Sub will be 12" Magnum D2

Box will be 3.1 cube after displacment of sub....tuned to 34Hz

how big should my port be and what differences would a 1" x 13" and a 4" x 13" make?? and why??

i should say: "If you want plans send me money"

But I'm feeling cheerful

For starters, 3.1 cubes is a little on the large size for a mag 12". nick recommends 2 cubes at 29hz (lord help me, i've built a million of those boxes).

Assuming that your box is 3.1 cubes after all displacement (sub, port, bracing):

1x13@34hz = 4.46"

2x13@34hz = 13.68"

3x13@34hz = 22.90"

4x13@34hz = 32.12"

More vent area often times equals more SPL, however, going overboard can kill your output as well. I think your best bet would be 3x13x22.90 (39 square inches). If you use those dimensions for the port, the box itself will have to be 3.75 cubic feet to make up for the amount of air the port will displace.