How tall do you have to be to ride a roller coasters?

It depends... Some Roller Coasters, such as the Deja Vu coasters at various Six Flags theme parks have height limits because of clearence issues. If that is not the case, comfort is also an issue. On classic Arrow looping coasters (Such as Demon at Six Flags Great America), leg room is not something that is very abundant. Also, you may have problems with your shoulder harnesses if your torso is long. The best bet would be to ride mostly on coasters with lapbars and ample leg room (B&M Hypercoasters are EXTREMELY good for this, such as Apolo's Chariot at Busch Gardens Europe) If you aren't sure, call the park ahead of time, and they'll be able to answer your questions.