Yes, I thought it was funny how the sizes went up by 2" except the 18-20"...Lola was about 4 months when i got her equafleece, i got it a little larger so that she had 'growing room' an *touch wood* she still has room at 6 months. She measured 16" so i got a 18-20" (they do that as a double size, not sure why as they do a 16", 18-20", 22", 24",26" etc-who knows!)
As for the trim, a puppy trim shouldn't be a problem now and might make it easier to keep her a (little) bit drier and cleaner!!
I originally got the 16" for betty at about 6 Months ..it was slightly tight so swapped it for the 18-20" which is a bit big. Betty is 6.5kgs and about 11 inches tall...it may give you and indication Sue..