My DD,6 is just at the 44" required for EE, Space mountain, and Mission:Space. She is very petite for her age, but is fearless. On our last trip, she was 4 and right at (or not quite, depending on the ride) 40", but she loved Star Tours, Splash Mtn. and Big Thunder Mtn. railroad. I know these are not in the same caliber as the other 3, but all I can compare it to, b/c that's all she's been tall enough to ride.
She has been reading the ride descriptions out of my Unofficial Guide, and is begging to ride these, but I have serious reservations. I have ridden SM, but not the other two, and wanted to know if they were appropriate for her to ride. Thanks for the advice!

If she is fearless she will love Space Mtn. but since you have ridden it you know it is a bit jerky. Mission Space can cause motion sickness because it "spins" to give you the feeling of blasting off and being in space. You cannot see yourself spin but both my hubby and son were quite sick after riding, you can definitely feel it if you are prone to that.

Expedition Everest is a family favorite of ours and my daughter was also 6 when she rode it. We thought the Yeti would scare her because he is quite large and moves towards you at the end of the ride. She loved it! I closed my eyes the first time and we got off and she said "Quick let's do it again!" My kiddos are also quite petite and skinny and have never been injured riding any of those rides. If she likes Big Thunder and Splash she would probably love EE's drop!

As long as you know her and what you think she would like and maybe talk about what the rides could be like and maybe get the Birnbaums guide to WDW for kids for her to look through and read together that might help.:)

I'd recommend doing the non-spinning/less intense version of Mission:Space, at least the first time you do it.

I think both M:S and EE are a step or two above Space Mountain - I think they're even a step above Rock-n-Rollercoaster.

We were in the exact same position. My daughter is going to turn seven this April and she is 46" and only about 40 pounds or so. My son, who is four, is almost as tall as she is and weighs more.

Anyway, she is fearless and adores roller coasters. Goofy's Barnstormer and a few at Universal Studios.

On our last trip we wondered about what she could handle. Personally, I think if she is like our daughter she'll be fine on all of them. But still, the hardest one is Mission : Space (spinning version).

Space Mountain, very mild (unless afraid of dark). Everest, nice and smooth and the drop is a scream but isn't a problem either.

But Mission Space has a queeziness to it. It's not scary, just that it can make people who normally do fine in coasters disoriented. Couple that with the claustrophobia and we simply scratched that one off our list.

IIRC from a physics standpoint being lighter they will have less inertial force at direction changes.