I am brand new at riding.I got a rossin to work out/ride to work on.I don't know anything about bikes.I have to replace the tires on it so I was going to try to go with something a little bigger.it currently has mavic ma40 wheels with continental 700 x 18c tires on it.what is the biggest tire I can fit on these wheels?thanks
MA-40's are now discontinued by Mavic. They were a great all-around rim in their time...replaced now probably by the Open Pro. Mavic "probably" would have recommended up to 28mm for the rim. I had a pair and eventually put 32mm's on them very successfully on trails and rough pavement.
18's are pretty narrow for most recreational riders. Your "ideal" width depends on your weight, your typical road surface, your rim itself, your riding style (how "quick" do you want to be), the tire pressure that feels good for you for comfort and handling.
Issues like tire pressure and sometimes width get to be debatable "Issues" for some riders.....first find what's comfortable for your present riding style/needs.
For faster recreational riding, probably 23 or 25's. For all around fooling around, and if you're a heavier rider, then 28's. Like saddles, you may want to experiment with width and pressure after figuring out how you will use the tire.