I'd actually say leave the cat on there. There really isn't much of a back pressure difference on or off.
I don't know the numbers off the top of my head, but start with about 1/4" larger than stock for the cat, and go to the muffler.
Have the muffler be 1/4" larger than that, and go another 1/4" larger the rest of the way out.
If you keep your stock manifold and down pipes, you shouldn't loose anything on the low end, and it will flow better for your upper end.
Just about what I did for mine, been happy with it.

Todd

1986 Pickup extracab longbed "Woody edition"
SAS - 33's, 5.29's, lockright rear.

02 IS300 SportCross - my "DD"
94 2wd v6 extracab
04 Outback Wagon- wife's/family car