32mm or 40mm waste for small bathroom sink?

[trick753]

Hi all,
I'm putting a downstairs toilet and wash hand basin in my garage and whilst working in civil engineering know all there is to know about underground drainage i'm not to sure about above ground stuff.
The sink i'm fitting is very small and will be used for nothing more than hand and face washing. I would prefer to use 32mm pipe as i'm running it inside a stud wall so more the more room the merrier, however if I have to use 40mm I will.
Many thanks for any advice given,
Trick753.

 

[Whitespirit66]

32

More of a basin then.

 

[trick753]

Many thanks, 32 it is then!!