I started on a small frame MTB (in the 90's) myself and eventually moved to a trials specific frame (monty x-alp mod). It was like night and day. If you want to progress, there will be many hours spent on your bike learning balance and what may seem like small moves. My feeling is, the sooner you can do that on a bike with geometry specific to whatever type of trials you plan to ride, the better off you will be. On the flip side, skills like balance and other non-bike specific stuff will carry through to any type of biking on any frame.