Noticed no responses in a week, so here is my best guess answer. Most mobos fit a "standard" size, first started by IBM, to provide interchangablity of parts from third party manufacturers.
The most common is nowadays ATX, or extended, with 3 to five expansior slots at the bottom.
AT
12" x 13.8". Based on the original IBM PC-AT's board. Obsolete.
BAT
Baby AT, a smaller version of the AT board. Obsolete.
ATX
305 x 244 mm. Stands for AT-extended - an enhanced version of the AT board. This is the most common form in use today.
microATX
244 x 244 mm. A smaller version of ATX. There are some nice small micro-tower cases that take microATX motherboards. If physical size is an issue, look at these.
Flex ATX
229 x 191 mm. Even smaller than microATX.
Mini-ITX
170 x 170 mm. Even smaller than Flex-ATX! These boards are tiny. Lots of info here.
WTX
A workstation/server version of ATX, with some more room. Not common