you are going to want the "Gen" sized ones. FYI, all the links we've posted will be UGC / CCG sized covers though which are too big for Genesis cases. If you are putting these covers in Genesis cases, you are going to need to resize them to 3188 x 2020 for them to fit correctly. This can be easily done in something like paint.net.
Also, for the 32x cases, the carts don't fit well in standard Genesis cases. For mine, I had to buy extra copies of the 32x sports games that came in clamshells and reuse those cases as the carts are slightly bigger. I think Master System cases might work also but the carts won't "lock" in.
Don't toss those cardboard boxes either! They are harder to find then the original boxes in most cases lol.