I recently acquired a trek 1220 from my local bike shop. I'm pretty sure it's a 96. It has all stock parts which is the shimano rsx groupset and mavic rims. How wide of tires can I squeeze under the rsx brake calipers?

What size tires are you running now and how much room do you have to spare?

The bike was sold to me with 700x23 tires however the bike had 700x25 stock. Im not sure exactly how much room I have to spare though

If you have the bike in hand and wheels mounted, it's really just a matter of measuring (whether with ruler or caliper). But I'd also measure the width of the tires you have on there now as what's listed on the side of the tire and what it actually measures once fully inflated on a particular rim might vary considerably.

+1 ^ Great advice

Measure clearance at stays and under brake calipers.

Yes to all of the above. It's not about the numbers on the tires. A tire with 25 written on it is not always bigger than one with 23 written on it.

I measured the tires and they are 23mm wide on my rim. I have about 10mm between the caliper and the tire. Could I fit 28s or 30s on this bike?

We don't have this kind of knowledge hanging around. You are the expert on how much room there is on your bike. Don't be afraid to try this. What's the worst that can happen?

Sounds like a larger tire would clear the caliper, but I have no idea whether it would sufficiently clear the chain stays or seat stays. No real way to know without trying or at least measuring in person.

It's 10mm clearance at chainstays and seatstays

10mm on either side of the tire or 10mm total? If the former, you're fine with 28mm tires (assuming they actually measure that width on your rims).

It's 10mm on both sides