No, all SODIMM's are not the exact same size, but are within a certain specific size standard as is listed here. Size is typically not the main concern, the type (DDR, DDR2, DDR3, etc), speed, CAS, etc are more important.
Your main concern would be to find compatible RAM though, using any of various methods online to determine the correct ones, Crucial's Advisory Tool, Kingston's Find the right memory webpage, and Newegg's Memory Finder would all be good places to start looking in conjunction with the documentation for your system.
Since you did not provide any specific information about the system you are attempting to upgrade, this is the best answer I can give you.