You can also change "Attachment Thumbnail Dimensions" under Attachment Options. I don't remember what the (too small) default was, but I increased it to 250px, which seems about right. Big enough to show reasonable detail, but not overpowering like full size images. Most users can see 3 or 4 thumbnails in a row on "normal" sized screens. And we used one of AndyB's add-ons to disable inserting full size images in posts - except for admins/mods. Oversize images in posts were driving us crazy.