I was just wondering whether anyone can tell me what the standard size of a Bonsai Tree is. Before coming to this website I thought that they were very small, plant sized trees, but it would seem after reading some of the posts you can get bigger ones. Could someone let me know?

The size of the tree really depends on the species, i've seen some big trees but I think that the most famous ones are the smaller ones for some reason

Most of the trees I have seen for sale are under a foot tall.

Most of the trees I have seen for sale are under a foot tall.
I know the ones that you mean, there are plenty of small Bonsai trees which as jippy said are probably better known that the bigger ones.

Here is an example of a small Bonsai Tree:

http://www.emilysplants.com/store/media/Bonsai%20-%20shogun%20juniper%20bonsai%20-%20image011.jpg

However Jippy also mentioned that you can have some very large bonsai trees as well:

http://www.carto.net/neumann/travelling/usa_california_2003_08/05_glendale_pasadena_huntington/51_bonsai_tree.jpg