By default thumbnails in 4 different Sizes, base size are shown below, aspect ratio is always kept when resizing so sizes may vary when using some additional features.
Small - 80x60
Medium - 160x120
Intermediate - 320x240
Large - 640x480
A page Excerpt, which is a full size snapshot taken from a section of the page, the default is a 400x150 page section, but the size and position is fully configurable.