Hi!
This topic was touched on a few months back without any resolution.
http://form.pbase.com/viewtopic.php?f=8&t=47139 And I noticed I sometimes see a thumbnail size pull-down with a choice of small or medium on the edit page, but its appearance was inconsistent. In addition, any attempt to select 'medium' instead of 'small' was lost when the 'update' button was clicked. I might speculate that such a feature is trying to come about, but it doesn't appear to be there.
I've not tried to mess with thumbnail size via CSS, I'm not sure where the custom CSS fits into the PBase gallery display page scripts; there may be some parts of that process that we mere users can't disturb. Maybe Marisa has some insights on the CSS issue.
I agree that having two or three choices for thumbnail size would be a nice tool.
DaveT