Hey there, my cutout is about 6" wide by 4" deep. Just leave enough at the top for the tabs to still snap at the top.
I just got under and marked it with chalk and cut it out with a jigsaw. I had to drill one hole of course to get the saw started. I smoothed out the cut with a rotary file in my Dremel tool.
Depending on the brand of hitch (mine is a Curt) I would think the size may be a little different so if you just mark it out with chalk it will be correct for your hitch. It does not have to really be perfect
since you will have to lay on the ground to see most of what you cut anyway.