As big as she likes. As big as a bus if she wants.
As long as you don't need to crop it very much, is all. https://www.damiensymonds.net/2014/03/how-aggressively-can-i-crop.html
Well ... yes, if the digitisation is of sufficient quality.
Will you be able to get the actual photo, so you can scan it yourself? This kind of work lives or dies by the quality of the scan.