It is all 2.5" ssd can be both used on laptop and desktop, don't worry about that. There will be holes for you to screw in to the hdd bracket, if you buy something say an intel 520 there are mounting bracket included.
I'm fairly sure most SSD drives are of the 2.5" variety. You can easily find a drive bay converter bracket to fit the smaller SSD if that is what your wondering about. I purchased a Samsung 840 SSD about 4 months ago and was probably one of the best purchases i've made regarding upgrading my PC.
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.
I think they may have had some 3.5 in SSDs, but it was a short lived idea.
Most cases now hold 2.5 inch drives as well as 3.5.
For cases that do not, adapters are made.
I personally like the ones that are 2 metal rails on either side(top right in image) because they can hold 2 drives AND can be used with ANY type of case. Some of the sled style adapters do not work on some case types(mostly older cases).