Hi Teresa, It definitely would be nice to be able to pick your sizes for both Collage proportions and Blank Canvas dimensions, and this feature will be introduced to Ribbet soon. As of right now, for a 16×20 blank canvas, use the dimensions 2400×3000. This is a perfect match for a 16×20″ collage at 150DPI (the proportions slider within Collage on the other hand can be less precise when trying to approximate dimensions, and being able to pick your size from a dropdown will resolve this).
