Minimum 550 X 900mm, unless a single family dwelling can be .362 sq meter, as long as no side is shorter than 500mm.
In English. 22 X 30, but can be 22 X 24. Took me an hour to research and do the math. The ceiling joists are 22 1/4 apart, 24 inch centers. I boxed it in yesterday afternoon. Every inspector has a different opinion about the rules. Second plumbing inspector wants expansion joints.
Hatch dimensions from memory. UK