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.