Well to be honest, any size bike is fine. People just go for what feels more comfortable. So you could get a 4ft person on a 26". So yeah it's down to personal preference. To be honest Onza are good bikes, but better for 20". If you look at the 26" they are quite bad. So if you can, go to an organised trials ride and ask if you can have a go on a 20" and a 26" and decide which feels more comfortable for you.
Rides and Locations Forum: http://www.trials-forum.co.uk/index.php?showforum=8
Have a look in there for some local rides to you. If they are not too attached to their bikes, I'm sure they will let you have a little muck about. Also if your a person who likes a light bike then the 20" are much lighter than the 26" only because there is more to a 26" obviously.
So my advice is to find out what size bike you want first and then ask about the bikes and the makes etc...
But if you went for a 20" I would recommend an Echo, or and Onza. For 26" I would recommend Rockman or Echo. I don't know much about 26" though as I ride 20". Obviously you don't want to spend £1000 on your first bike, because it is going to get scratched to pieces.
The for sale forum: http://www.trials-forum.co.uk/index.php?showforum=3 - Look here for pre-owned bikes if you do not want to spend out on a brand new one!
The wanted forum: http://www.trials-forum.co.uk/index.php?showforum=41 - If you haven't found what you were looking for then request it in this forum.
Hope this helps,
Danny.