changing resolution of finished mapping product

Look at the GDAL tools. You can do it with QGIS which is just a nice front end to GDAL. You want to "translate" the GeoTiff created by Maps Made Easy to the desired GSD.

Example of how to half your GSD:
gdal_translate -outsize 50% 50% full_gsd.tif half_gsd.tif

If you want bigger GSD, you need to fly higher, if you want smaller GSD you need to fly lower.