Hey Kareen,
The size of your sump is determined by three things. First, if it will physically fit, which can be a big consideration. I've had stands that are just too narrow to fit anything inside of them. Second, it needs to be of large enough capacity and height that the pump (and any other equipment) remains properly submerged at all times, and you're not constantly topping it off. Third, it needs to have sufficient empty volume that when the pumps are off, and the pipes and plumbing drain into the tank, it doesn't overflow (e.g., during a power outage).
Those set up the minimums for your sump size. Obviously, you can plumb a 5,000 gallon swimming pool into your 2.5-G and use it as a sump -- there's no maximum.
I've seen tanks where the sump has exceeded the display!
Your plumbing and such will determine the third criteria. The less pipe you use, the better your air breaks, etc., the better you'll be. On my installs, the water increases less than an inch. You can mitigate top off with an automatic top off device, but that's not critical, especially in a freshwater tank where you're not apt to be running a lot of level sensitive equipment.
Lastly, if you're using bioballs or some other sort of "wet/dry" media, you do need to have enough room that they're properly wetted and not fully submerged.
I think the 30-G is appropriate for this.