What exactly is a "trials" bike?
Trials bikes were originally just mountain bikes, at the time most mountain bikes had 26" inch wheels and 135mm rear dropouts (trials is basically shorthand for mountain bike trials). Trials bikes of this design usually fall into the "stock" category.

When people started modifying bikes to be even more well suited for obstacle courses, the 20" wheels and 116mm rear dropouts were introduced, this is when the bikes started to closely resemble bmx bikes. Trials bikes of this design usually fall into the "mod" category.

"Stock" bikes and "mod" bikes make up the two officially recognized categories of trials bikes, so, plan for 20" wheels or 26" wheels. The 20s are generally more nimble but the the argument for 26s is probably more clearance and better balance.

There's quite a bit more information on the subject:

https://en.wikipedia.org/wiki/Mountain_bike_trials

http://www.biketrials.com/intro/what.shtml