+1 on the push-pin adjustment. Five millimeters makes a HUGE difference in seat height, and those things usually adjust in increments of 20 mm or more. When you're small, it's an even greater issue, because the increment is a greater percentage of your leg length.

The other thing about those bikes is the crankarm length. Often they come with non-replaceable cranks of 175 mm or even longer - again, way too long for most smaller people. I can't ride spin bikes at all without hurting my knees. (Which is a perfect excuse for not having to learn to teach cycling at my gym. )