Franz - "Outside corner to outside corner". Do you mean with or without the skirting (or whatever you call the boards that are nailed to the outside of the poles for the metal to attach to)?
WVBill - "If you set your poles so that they are 24' outside corner to outside corner then you add 2X4 girts you will end up with 24' 3" of surface you need to cover with your sheathing/siding on two of your walls". OK, this part makes sense. But, you lost me on the part where you said "(or 24' 1 1/2" on all four walls)".
I think I have it now though. I need to set the poles 23' 9" apart outside to outside, so when I add the skirting (the boards the metal siding will nail to), then the dimensions from the outside of the skirting on one side to the outside of the skirting on the other side will be 24' even.