Originally Posted by Cyclesafe

The tire size dimension usually most relevant with fenders is one never published by manufacturers - height. Measure the smallest distance from your 32-622 Marathon Plus to the fender. Then subtract the diameter of the largest piece of gravel that you think might be kicked up by your tire and catestrophically lodge between the tire and fender. If there is any room left, that's the amount higher your new wider tire can be. Front fenders usually come with "break-outs" that will detach the fenders before something seriously lodges, but rear fenders don't.

Obviously you can fit a taller tire if you don't use fenders. So you might have to chose between traction and cleaner drivetrain. You'll have to try different tires: Isuggest buying a few from a retailer like REI which has a liberal return policy "if the tires don't fit". Also, some fenders may allow wider tire than others. Again, you'll have to find out which ones are best for your bike through trial and error.