But the Available Power may be an issue if you look at a Unpowered HDD Enclosure. It's better to get a Powered USB Enclosure that has it's own Power Supply as this removes the possibility of doing any harm to either the HDD/Enclosure or Computer where there isn't enough power available.
A 3.5 Inch Drive requires 12 Volts to run it while 2.5 inch drive requires 5 Volts If I remember correctly. So you can use an External Power Supply for the Enclosure and depending on what is required by you this will dictate the type of Enclosure that you need to buy.
The 2.5 Inch are very portable but the down side is that they are small, light and easy to knock off a flat surface and destroy the drive inside the case. A 3.5 inch drive is more bulky and less portable as it comes with more than just the Enclosure and USB Lead there is a Power Supply as well which is generally a Plug Pack type Supply.
The 2.5 inch drives tend to have 2 USB Plugs on them for extra Power and most can be fitted with an external Power Supply which may just be a Lead that plugs into a PS2 socket or a Plug Pack depending on what if anything is made by the enclosure maker. These tend to be Optional Extras these days but it really depends.
Now what you should know about External HDD Enclosures is that they are not Full Time use Items as they don't have anywhere enough surface area to shed the heat that the drive inside it produces. When a HDD overheats it performs badly and the high temperatures shorten the life of the drive causing them to fail more often that internal drives do. Then because they are small and easily transported they are also easily knocked off surfaces while they are running and a small jolt while a drive is running can destroy the drive.
Col