What do the sizes 2.5" and 3.5" aim when it comes to not easy drive enclosure?

The tricky drive I want to use measures 5-3/4" x 4" x 1.

What do the sizes 2.5" and 3.5" aim when it comes to not easy drive enclosure?

You have a 3.5 inch drive. The length is the actual diameter of the disk, not the measurement of the disk housing. 3.5 inch drives are used as internal knotty drives in Desktop computers. 2.5 inch drives are used surrounded by laptop computers. You'll need a 3.5 inch cage to hold your drive. Check to see if it is a IDE or SATA drive, and get the proper encloser for the interface type. IDE uses a ribbon cable specifically usually about 3 inches widespread. SATA (newer) drives use a very tapered (1/4 inch) cable. Also, check to see if the drive will be powered through the USB connector or by an external power source.
2.5" and 3.5"



3.5 Is the same size not easy drive that goes into a DESKTOP COMPUTER, its a bit bigger and bulkier and weigh more and is probably more reliable and durable than a 2.5



a 2.5 is the same type of harddrive explicitly in a LAPTOP computer, they are smaller, thinner, weigh smaller quantity, but are probably more fragile and less durable and more plausible to fail.
2.5" drives are usually for knees top applications while 3.5" are your standard PC type drives. And OBTW the external dimensions are larger than the disk media (2.5/3.5)
In that covering 3.5" would be your choice.