Exposure follows the "square law" vs distance. (i.e. Divide the new distance from lens to paper by the old distance, then square the result, then multiply the old exposure time by that to get the new time.) If you double the distance between the lens and the paper, you must quadruple the exposure. You will have to make a test exposure at the new time and distance, since papers suffer noticeably from reciprocity failure.
The easy way to handle this, as noted above, is to use a meter. Take a reading in a key area (preferably about zone VI in the print), then change the enlargement size, read the same area again, and adjust the aperture until you get the same light level as before.