The PICMG COM Express module specification revision 2.0 describes three sizes of COM Express modules: Basic, Compact and Extended.
Basic modules are the most common size and measure 125mm x 95mm. To meet the demands for smaller form factors, Compact modules are small, measuring only 95mm x 95mm, similar in size to PC/104 modules. Extended modules typically offer advanced features and measure 155mm x 110mm.
Driven by the demand for even smaller sizes, the Ultra form factor measuring only 55mm x84mm is under consideration. Depending on the required features, custom sizes are also available from COM Express vendors.
Additionally, there are six different Types that define various pin outs. See the COM Express Types FAQ for additional information.