The Raspberry Pi is available in a couple of form factors. There is the standard Model B (85.6 x 56.6mm) with a full complement of I/O connectors.
If you are willing to give up some features, you can get the squarish A+ model (65 x 56mm), the Compute Module form factor (67.6 x 30mm), or even the ultra-compact Pi Zero (30 x 65mm).