TooWaBoo et al.,
Thanks for the quick reply. Duh, the -/+ buttons! So, I've adjusted those, cleared the cache, quit, and experimented with removing the left and right panes (histogram/history and editing tools), and the thumbnails have indeed grown larger (especially with the cache clearing, thank you!). But their corresponding spaces have grow larger apace. It still seems as if the thumbnails are around 1/4 the size of what they should be. Note in the photo below how the crops are seemingly placed where they should be if the thumbnails were much larger. (The crops are fine in the editing window and in jpg outputs, btw.)
I believe something (in addition to my ignorance) is still impeding proper display here.