Wow, there are some really complex ways of dealing with this out there!
Originally Posted by johnnywalker
The method above is probably the easiest way, just remember you need to measure the illuminated area on the baseboard not just the area cropped by the easel blades. The ratio of the two areas gives the factor to change the exposure time by (assuming you leave the lens aperture the same). As others have stated you will likely need to increase contrast (1/2 to 1 grade typically) if it is a large increase in enlargement ratio.