Hey buddy, nice to see another canadian on the site. You ride so much like craig robinson it's crazy. I commented on your thread on MTBkanata.com
Ok, well, you should borrow alot of ideas from BMX geometry because you'll be riding alot of BMX inspired stuff, but you still need to keep some trials aspects.
For the head angle, the steeper it is, the longer your top tube is, which makes the bike harder to pull up. The slacker it is, the longer your wheelbase is. Therefor, I'd say a 73 degree head angle is good. It's close to the 74 deg average BMXs have and you'll be able to maintain a 1010 wheelbase without too long a top tube. The steeper you go, though, the easier front brake tricks, spins and control will be.
A shorter wheelbase will make spins really easy. 1010mm is really good. Don't get too long a fork though. It's better to have a high handlebar set up than a long fork. My bike's gonna have a 1010mm wheelbase but that's cause I want it to feel like a BMX, you're alot taller than me, so you might be able to pull off 1020, which would make trials easier.
As for chainstays, I think the length on the Leeson and the Ashton are around 380mm, or 14.9", but I'm not sure. Copy their length. It's worked for everyone. Because of the smaller wheel, you do need to go shorter than what you would on a 26" wheeled bike.
I'M gonna be getting mine made by Joe Mamma's in Ottawa this winter. If you have anymore questions or just wanna come down to our nation's capital to ride, PM me your MSN address. We'll talk.