Depends if you're talking about just a sump, or a sump/fuge combo. The sump can be any size you want it to be. As long as it can hold the equipment you want it too and has the additional room to handle the extra water when the return pump is off, it would do. There is no actual rule of thumb for a sump. However, for a fuge there is a general rule of thumb on size, but varies greatly depending on who you ask. For a fuge, personally I think the bigger the better. Flow would be dependent on what your overflow can handle and flow for a fuge would be dependent one both what the overflow can handle and on the fuge size.