Quick question

Well to give a bit of background I am doing an art gcse at school and our final project is to do with ourselves so me being the rs addict I am have brouhgt rs into my art.

I already ahd the idea of doing a pixel image for this [project and my teacher said it would be a good idea.

So my questions is:

does anybody know the dimensions you would need to use to make a pixel image (possibly eanlrage it to bigger size) without it going well pixelated for it to be around the size of a5?

(apprently in america these paper measurements aren't well know os a5 is half a4 whihc is 'printer paper')