I’m new to this hobby, but I imagine this will be a tough question to answer without knowing your specific goals. Sumps can be used for a plethora of purposes.
Are you looking for simply a way add volume and for a place to put your heaters?
Do you want room for a refugium for macros?
Will you be putting a protein skimmer in it?
Some people use sumps for breeding fish, a place to connected reactors, hospitalize corals or injured fish, a time-out zone for aggressive fish, copapod breeding, extra biofilter media, and so on and so on.
What do you want out of your sump?