I have Marin four corners bicycles and I want to switch to 203 rotots but I dont know If the frame and fork can accommodate larger rotors.

Why? Because my weight is 100kg (220ib) + 20kg (44ib) of gear + water. And on long downhills they are not enough for me.

Maybe 180 rotors will work as well.

You will need to contact the manufacturer.
I checked the owners' manual to no avail.

180mm should be fine.

Just a note, I don't know of any frame failures due to rotor size.
I DO know of frame failures due to crashing from brake fade.

The roter diameter is determined by the location of the mounts and the calipers themselves. Some systems allow for adaptors or alternate mount position to accommodate different rotors.

So start by checking specs, and doing some careful measurements.

Right. You get into the realm of experimentation when you start adding spacers and adapters. Plus it's possible that in so doing you introduce the dreaded "hysteresis" because the caliper's load path to the frame or fork now has some sloppiness in it.

Before going to a larger rotor I'd definitely exhaust the possibilities of pads and possibly bleeding the brakes (I'm assuming hydraulic)

but what do I know ?

/markp

BTW before you start spending dough, think about what you're trying to achieve.

A larger rotor offers 2 advantages, but don't expect a sea change.

1, by virtue of both greater mass and surface area larger rotors offer more total brake heat capacity within a short time frame. Useful for long, steep descents, and possibly urban riding, but no benefit otherwise.

2, by virtue of the diameter you get more braking torque for a given lever force. This may help if you feel you have to s was ueeze harder than you prefer. But pad friction properties vary, so possibly a pad switch can offer the same benefit.

So, think about what benefit you may realize, and whether it's enough to justify the cost for your purposes.

Have you tried reducing your max speed a few MPH?

In a panic stop, the front is going to do nearly all the work, so I'd "abuse" the rear and only use the front lightly while descending.

Don't continually "ride" the brake. It doesn't have a chance to cool. Use somewhat firmly and scrub off a few MPH or not at all.

I grabbed this off Jenson for the 2023 model. I assume it has 160mm rotors. Look close at the bracket that attaches the calipers to the fork at front and stay at rear. It may be labeled or stamped with the rotor size(s). Flipping the bracket may allow a larger rotor. My Shimano and Techtros are. TRP if you have those may offer alternative brackets for the rotor size. You may have to remove the bracket to see the marking. The TRPs I saw were not marked on the side. Performance Bike lists a few to show how they are marked. I noticed the 140/160 but the largest was 180 and had no option to adapt to a larger disc.

If you have a 2022 bike with Techtro brakes see if the are correct. Call or email Jenson. They sell Marin.

https://www.jensonusa.com/Tektro-Disc-Brake-Adapters-203mm-Front-51mm-IS-Caliper?loc=usa&gclid=Cj0KCQjwib2mBhDWARIsAPZUn_mhOKM-xhPM8_wYu_d9CVXXrCnL_e3-z1EviS0VGBX-SCs_KlK8_CwaAsvpEALw_wcB





https://www.performancebike.com/trp-disc-brake-adapters-black-fm-fork-pm-caliper-160mm-front-abad000058/p1391499