It comes specified by opening size and sheet metal gage. Mine was 3/4" opening (pretty common - that is the long dimension of the diamond) and I think 9 ga. That is a perfect size for a brush guard, IMO, especially after using it, and very common. You can't go wrong with this type. BTW, you want the type called "flattened". They roll it after doing the punching and expanding so it lays flat and doesn't have quite the sharp edges of the raised type.
A local supplier may have "drops" or leftovers from previous customers cutting down a 4x8' sheet (standard size). If so, they will typically give you a better deal on that, so ask if they have drops first. You might get lucky and they may have a piece big enough for you.
A quick web search on those terms will tell you a lot before you walk into the suppliers.