I agree with Carl. I don't think leaf size is absolutely essential with Japanese Maples either. Providing the leaves aren't ridculously huge, you can indeed produce very convincing bonsai, which are pleasing to the eye.
Aside from their stunning autumnal colouration, Japanese Maples are equally as beautiful when viewed during the winter months, when the shape of the tree and the ramification can fully be appreciated.
Uncannily enough, I too am planning on trying new and different cultivars for suitablility for use as bonsai. With over 400 to choose from its a bit too tempting not too!
As I am only really just getting into Japanese maples, I recently came across a couple of cultivars that are (so I read) apparently pretty rare in the UK. How true this is I don't know... but here they are regardless
This one is called "Orange Dream" and I absolutely adore the colour and the shape.
This one is "Orido-Nishiki". Lovely colouration and foliage again. I would really love to try these out and would like to know if anyone else has attempted these cultivars and what degrees of success they have had.
Also, If anyone has a link to a list of Acer Palmatum Cultivars with pictures... (if one exists) I think it would be a very handy addition to this thread.