The 2 main factors in choosing the right trials bike are riding style and height.
With the recent changes to UCI rules wheel size no longer restricts what events you can compete in. Anything from 18" to 26" is legal. (luckily not 29ers though).
There are some advantages with certain moves on certain wheel sizes but overall these are negligible and down to personal preference. However many people do find a 20" the best bike to learn the basic moves on.
In regards to riding style you have your competition riders (google Vincent Hermance, Benito Ross, Kenny Belaey) and your street riders (google Danny MacAskill, Ali C). Of course you can cross over between disciplines but your first decision should be what type of trials you wish to emulate. The geometry between the bike that Ali C rides and the bike Vincent Hermance rides is radically different but they both run 26" wheels.
In regards to height it is not set in stone but basically if you are smaller you are more likely to suit a smaller bike and a taller rider a bigger bike. This often means smaller wheel sizes for smaller rider but it can also just be a bike with a shorter reach (bb to headtube).
For the record I am 5'10" and riding a 24" competition bike and a 26" street bike and both are set up very well to suit my height.