wow you have your work cut out for you.  i would be trying to get some parental assistance when the teams are big as there really is too many for one coach to give her attention to skill development and adjustment in large groups.

but when you do have large groups break them up into 2-3 main age/ability/teams.  there are a number of drills that you can do like 3-5 person weaving, passing, skill drills using ladders, balls, hurdles, cones, obstacle courses, that they can all do, and then you can get a drill that has a mulitple of progressions that you can start the youngest or less skilled group at and then have the others do a progression of the same skill.  you can also play 2/3 on 2/3 which is pretty straight forward, half court games, shooting practise, defence drills starting at the basic level for those who are at that point (ie in front defence) and move onto the more advanced side on defence and blocking/zoning.  

netball is one of those games you can incorporate many skill levels, but it is a much harder job.  maybe a letter out to parents or players to help with the coaching might be in order to get some interest and some help.

good luck with it.