new crawl space access?
Most of the jurisdictions in the U.S. use a version of the International Residential Code published by the International Code Council (ICC). You can call your local Building Code office to confirm the minimum requirement and if you need to purchase a Permit.
The Code requires crawl spaces through the exterior perimeter wall shall be 16” x 24”. (See ICC R408.3) The code doesn’t indicate which way the 16” is required, vertical or horizontal. If the opening extends below grade, then you’ll need to provide an opening like a window well too.
Most exterior walls are load bearing, so you’ll need to reinforce the floor with a header for the part that rests on the opening. However, if you choose a side of your house that has floor joists running parallel to the opening rather than perpendicular, you may be able to skip the header.
I’d contact a local carpenter or general contractor to do the work. (Make sure they are licensed in your jurisdictions.) If you don’t know of a good contractor, I’d contact a local architect and ask for a recommendation.