Most tires these days will have ETRTO (ISO 5775) size markings in addition to the archaic and confusing markings. 26" tires are particularly problematic with the archaic markings, so whenever possible look for the ETRTO size. ETRTO sizes are given as two numbers separated by a hyphen, e.g. 38-590. The first number is the nominal width in millimeters, the second number is the bead seat diameter in millimeters. The bead seat diameter must match your rim, otherwise the tire will not physically mount on the rim. The width is a little more flexible in that it is limited by the clearance your frame offers. If your old tire has ETRTO size markings, matching a new tire to those sizes is the safest bet. Look at how much clearance there is between your old tire and the frame; this will show you if there is space for a wider tire, should you prefer that option.