I wouldn't make the hole square...
it can be longerlong the direction of the joists,
making it easier to get in ind out...

and, as someone suggested,
can you squeeze between the existing joists?

I dont know if i can get through a foot wide...

depends on how high the roofspace above where you put the hole is,
I guess the more height above it the better,
and a rope hanging down to help pull yourself up...

[g]