Reddit reviews It's Okay To Be Different (Todd Parr Classics)

I'd go with a casual, affirming response like "Yep, that lady uses a wheelchair to help her get around. Some people use a wheelchair because their legs have trouble walking." or "Yeah, that man has a different skin color than you do. Isn't it neat that skin comes in so many lovely colors?"

You can also try to make sure your daughter is exposed to a diverse cast of characters in books, TV shows, etc so interactions with "different" people in real life won't be as surprising. For a 3yo, Todd Parr's It's Okay to be Different would be a great place to start! Also check out this book list from the School Library Journal.

EDIT: Just came across a super relevant new book - Who We Are: All About Being the Same and Being Different

The only book I can think of is called Don't Cry, Big Bird BUT I would ask you to preview it to see if you think it's acceptable because I don't remember all the details and I don't want to be held responsible for traumatizing your cousin!
From what I can remember, it's a picture book where Big Bird is very sad because he's too tall to join in games (jump ropes are too short, etc.). He's all sad about it, but then Mr. Snuffleupagus, who is also really big, says that he likes Big Bird's size. He helps Big Bird modify games (tying two jump ropes together) and eventually Big Bird likes his size and finds it useful-- he rescues his friend's kite from a tree when no one else can.
The reasons I can remember for not recommending this book wholeheartedly-- I believe they use the word "big" instead of "tall." It seems like a little thing, but for a girl who is already feeling self-conscious, it might not be the best choice of words. (And yes, no one should be ashamed of their body, regardless-- but I know that personally, when I was called "big" as an underweight but tall preteen, it stung.)

Another resource-- not specifically for tall girls, but adhering to the theme of loving yourself for who you are-- I found this list of "Positive Princess" books really helpful when I started nannying for a very appearances-centered kindergartener. She loves princess books, but a lot of them weren't sending a very positive message. These princess books focus more on having a healthy view of yourself rather than idolizing some airbrushed "Princess" character. I haven't read all of these, but Jane Yolen's and Heidi Stemple's Not All Princesses Dress in Pink is phenomenal.

More awesome picture books celebrating differences and diversity (Amazon has most of these for less than $3 used, and they're mostly popular enough that I would think your library would have them):
It's Okay to be Different
What I Like About Me
I Like Myself!

To take a different approach-- I know this sounds shallow, but when I was still growing and way more self-conscious about my height, I LOVED the fact that Nicole Kidman was (well, is) 5'11" (I was obsessed with Moulin Rouge). I ordinarily do not care about celebrity...stuff, but I thought it was so awesome that she was the same height as I was! Depending on what your cousin's interested in, consider showing her some pictures of tall girls to boost her confidence. For example, if she's familiar with the President's family, Michelle, Malia, and Sasha Obama are all quite tall (Michelle and Malia are both 5'11", and Malia's only 14!) The Obama girls are gorgeous and stylin, and I know I would have loved to see photos of tall younger girls when I was a kid (as opposed to tall women). Hell yeah, tall first family!
5 is too young for The Hunger Games, but Jennifer Lawrence is 5'9 and taller than her costar. She also seems to have a fairly positive view of health/body image as well.
There are a lot of tall women in the media-- Queen Latifah and Tyra Banks are 5'10, Taylor Swift (if she's into her music) and Karen Gillan (I love Doctor Who too much to not include her) are 5'11... and that's just a start! Look at Olympic athletes (Missy Franklin is only 18, 6'1, and a 4-time gold medalist) for other tall role models!

But at the end of the day, more than any books or media, you are the one who will have the most positive impact on your cousin! Make sure to talk about your own height in a positive manner, don't set a bad example by saying anything negative about your own body (or anyone else's), and be sure to mention how much you like being tall when you're around your cousin. I outgrew my mom, sister, aunts, uncles, and all my male and female cousins as a kid-- I would have loved to have a role model like you! Your own personal relationship with your cousin will be by far the most powerful and long-lasting influence on her self image.

It's OK to Be Different is one of my favorites:
http://www.amazon.com/books/dp/0316043478

Not specific to being tall, but a goofy book about being different. For a younger audience than her, but still lots of fun :)

This book has been a family favorite for YEARS since one of my kids found it in the school library. We've worn out at least three hardcover copies at this point - I wonder if your daughter might like it, too. ❤

We've discovered the Rookie Toddler series at our library. It's a great educational series that our LO loves! https://www.amazon.com/Books-Rookie-Toddler/s?ie=UTF8&page=1&rh=n%3A283155%2Cp_lbr_books_series_browse-bin%3ARookie%20Toddler

We have 2 Peekaboo Sophie books with nice heavy flaps.

Who's Hiding? is also a good lift flap series. We have this one https://www.amazon.com/Whos-Hiding-Garden-Lift-Flap/dp/0764163159

I like the books by Todd Parr too. We have the I Love You book, the Daddy book, and Doggy Kisses. Lots of bright colours and fun to read. I want to get this one next https://www.amazon.ca/Its-Okay-Different-Todd-Parr/dp/0316043478

Goodnight Moon and It's Okay To Be Different. Anything by Todd Parr, really.

I do an activity using the book It's Okay to be Different. First, we read the book, then the kids make a person out of contruction paper using a math glyph format by answering questions about themselves.

If you live with one parent, you get a square head; if you live with two parents, you get a triangular head; if you live with no parents, you get a round head.

If you have a pet, you get a red shirt; if you do not have a pet, you get a blue shirt.

If you like math best, you get green pants; if you like reading best you get yellow pants.

The number of eyes is the same as the number of children that live in your house.

The pictures turn out really cute, and all my kids from pre-k through 8th love it because it's funny.

If you're interested I can probably email you the pattern I use to make the people tomorrow night. PM me your email address if you'd like it.