What’s commonly referred to as a 3.5″ drive has a platter size of about 3.74″, so 4″ form factor is about right IIRC.
It’s known as a 3.5″ drive because it fits in the same drive bay as a 3.5″ floppy drive. The floppy disks are 3.5″ so the drive is a bit bigger than that.
Dunno as it’d be much cop for an Xbox One drive though, it’s not very big (capacity wise) and will probably be painfully slow. (And AFAIK, you can’t add an external drive to a 360 in any sort of useful manner.) You’d be better off getting a nice USB3 job for the purpose.