In general the height requirement is around 4ft, especially for the big roller coasters . But there are rides geared towards small children that have an much lower requirement.
It depends on the particular ride.. they also have a test seat before the ride so that one can check if they can go for the ride or no..
Their official site will tell you the height requirements. However, Universal Studios is reallyed geared for older kids. Im always sad to see the disappointed kids that do not make their height requirement. Their staff is quite strict with the height requirements.
Thanks! Your answer is awaiting moderation.