What do you have in there now?
In general, you want about 4mm of space/tire clearance, to the to the sides and the top. A quick way to estimate what you can get away with is to grab your 4mm Allen wrench and use it as a feeler gauge around the tires - check the sides at the chain and seat stays, the top at the seat tube and brake caliper, etc. That should give you a good visual on how much more you can fit in there while still maintaining enough clearance for debris and whatnot.
Edited to add: also keep in mind that some tires will measure different than others - a nominal 25mm from some brands might measure out to 28mm in real life, so the above method is best when you're staying within a certain manufacturer/model.
Last edited by WhyFi; 04-15-20 at 01:10 PM.