For a USB Type A. I went to mouser.com and found a data sheet for a stardard type A connector from TE.
Looking at page 4 of the there is a recommended cut out size of 13.55X6.00
Looking at page 3 the shell is (12.5+-.1 x 5.12+-.1). If you want to clear just the main shell the largest it can be is (12.6x5.22) You can get away with the cutout being 13.55+-.5 x 6.00 +- .5 which means the smallest the cutout can be is 13.05x5.5 which will clear the shell.
If you also need to clear the tabs:
Max shell width: 12.5+.1 = 12.6
max shell height: 5.22
Max tab: 1.065+.105 = 1.17
So max width is 12.6+1.17*2 =14.94
and max height is 5.22 + 1.17*2 = 7.56
Say you always want 1mm of clearance around the connector and +-.5 tolerance -> 1.5
So your cut out should roughly be 16.5 +-.5 x 9.1 +-.5
Not knowing the manufacturing process +- .5 is just a rough estimate but i think a pretty safe one.
Overall you want to look at the mfg drawing and find the max dimensions the connector can be. add what you want for clearance to that then add what you want the tolerance to be. This will give you the nominal size of the cutout with the tolerance that you chose.
Max connector dimension + clearance + tolerance = Nominal cutout size