That just depends on the person and what their greatest fear is. Tower of Terror is the scariest if you hate big drops and falling. Rock n' Roller Coaster would be the scariest if you are afraid of high speeds and/or going upside down (it's the only ride at Disney that has inversions). The more intense "orange" version of Mission:Space would be the scariest if you hate spinning and get motion sickness from it.
It depends on the ride. I'm going to say no, but see if you can find the official answer at www.disneyworld.com. Even if you do try to go on a ride you shouldn't be on, Disney has placed signs saying not to ride this ride if you have...(there is a list of health conditions.) THis goes for most rides at WDW.
probobly Rock'n'Roller Coaster, Expedition Everest, or Tower of Terror... It all depends on what you think is scariest. If the psychological free fall ride scares you, then avoid tower of terror (although the drops really aren't that bad). If it is inversions then avoid Rock'N. And if it is 'ricketyness' or intenseness then i would say Everest.
It depends on the ride, but the highest height restriction I have ever seen on any Disney ride is 48 inches (Rock n' Roller Coaster at Disney World and California Screamin' at Disney's California Adventure both have that height restriction). Most of the other ones allow smaller kids to ride, with height restrictions usually being somewhere between 36 and 42 inches.