the optimal resolutions are binary powers, e.g. 1024 x 512 will work well, but 1000 x 600 will get resampled and won't look as good. iirc the max resolution that can be handled is 4096 pixels in either dimension, larger than that won't load. not sure if there's a Mb limit too.
can't comment on bump maps specifically as i haven't used those so much, but the same general rules should apply.
on the question of removing unused textures, i was
going to say you can work down the list in the Texture Selector window and hit 'Delete' for each one, and it should warn you if you try to delete one which is being used anywhere in the project.
however, i went to test this just now and it happily deleted everything without giving any warning, which i think is a bug -
@Steve there should be a 'there are references to this texture' warning, right? or am i losing the plot? build 1402 fwiw.