Typically, the hotswap drive needs to be essentially identical to the drive it is replacing, but every system is different.
Some systems can support swapping in a bigger drive with similar characteristics, with the excess space being unusable. Certainly, it can't be smaller than the drive it is replacing. I imagine that the tolerance for similar but bigger drives would be discussed in the technical specs of the system, or can be obtained from the company support mechanisms.
One of the very unique features of the Drobo system is that it can handle size and flavor of drives.