It depends on what you want to store on the HDD. If you want to collect mp3s or video clips, it is easy to fill 4GB quickly. But for storing text files, 4GB will be enough for decades.
What you should consider is that the Amiga cannot handle HDDs bigger than 4GB without software patches. So if you don't want any hassle, you should stay smaller than 4GB (4.3 billion bytes). Also a HDD needs some memory for buffers. The bigger the HDD (and the more partitions) the more buffers you need. If you don't have a memory expansion, you should surely stay below 4GB.
Bye,
Thomas