A quick summary to help you choose your needles based on your projects:
Very fine fabric (muslin style) size 70/10 needle
Fine fabric (cotton voile, poplin style) size 80/12 needle
Medium fabric (cretonne style) size 90/14 needle
Heavy fabric (twill, basket weave style) size 100/16 needle
A test is recommended on a scrap of fabric before beginning.