Its just a place for hiding equipment and increasing water volume so I'd say you can have whatever size you want, it doesn't need to be huge. Most people have a larger sump for a larger tank because of the footprint it creates underneath the tank.
I bought a 70gal for my 180, it was the right price at the right time. I didn't want to go over 250 gallons total volume though and considering neither tank is truly full, I'm at about 220 gallons. I was thinking of waterchanges at the time and what I could reasonably get done.
I've got a 90 gallon sitting here, you just need to put baffles in it (or not, I run my sump without baffles). It does have a hole in the bottom though that would need to be covered with a glass plate or something. I'll give you a good deal
