Most SSDs come in a 2.5" size format. Within this format, there are varying thicknesses of drives. If you are putting it into a laptop, you want to figure out what thickness of a drive your computer can accept.
The other common SSD sizes are mSATA format. They are little cards that are commonly used in ultrabooks, the new mac book pros, and other space contrained devices. Their prices are on par with regular SSDs.
Lastly, there are PCIe SSDs, but those tend to be very expensive.