Ok, dont really know how I'm going to start this so I'll just get on with it..
Chainstays: Back in the day when trials was ridden on XC bikes the stays would have been around 410mm, nowadays they are mostly 380, 375, 365mm. Having short stays makes it great to get up on the backwheel and control when your up there, but I think it also has some major disadvantages. I think slightly longer stays of around 390 or 385 are more benificial for gaps and ups as they give you more leverage.
BB rise: Again, back in the day, we were looking at -20bb, now on most frames we see +10, +20, +30, +50, +80. This, I feel, has a great advantage for control on the backwheel and makes everything feel smaller as your up higher, but I also think it has some major disadvantages, such as you cant drop the front as much in gaps, so you loop off, the fact that it makes the bike ride shorter so you can end up being more hunched on a bike with a longer wheelbase.
Headtube angle: 71 or 72 seems to be the most popular, but why? To make the bike FEEL shorter yet RIDE longer, hmm is this a good thing? I dunno, but I think steeper angles would have more advantage, they would bring the backend up quicker in sidehops, ups to front wheel etc.
Wheelbase: Short = flicky, long = more control for static power moves. Cant really say much more for wheelbase to be honest.
So I wrote this kinda quick, but maybe you understand?
All this might sound silly but I recon there is truth in it.
So, MY ideal frame would have to be;
Wheelbase: 1065mm
Head angle: 73.5deg
Chainstays: 390mm
BB rise: +5mm
OR, you could just get on a bike and go ride and not worry about all this tripe B)
Sam