whats the minimum loft opening size?

I've got 18" inbetween the roof beams is this enough for a loft opening with ladder? I could make the width 18" or the length and the other dimension I could make 30".
I wouldnt be able to line the hole (coz that would make the opening 16")so the archatrave would have to be fitted to the roof beam.

I hope you are thinner than me

you could fit trimmers and go up to 38"

Yeh I ve thought of that but its my sons house and he's worried that I'll have the roof in if I cut out one 600mm section.
For a 2 section ladder the width required is 406mm and 437mm for 3 section. It means on the long sides I will only be able to line with 6mm mdf and nail the archatrave to the roof beams rather that the liner.