... yep, done this recently. We compared 15m Naish Fly, 16m Dyno, 17m Edge and 18m Dyno.
The 15m "looked" surprizingly big for a "15m" and the differences are less than you can see by mere laying them on the ground but as Plummet says, it more important the range it can get you. If you could get a 12m to cover the range of an 18m then I'd have a 12m
this one is 18m on the bottom: 16m then 17m then 15m on top
this one is same as above
again, bottom up 18m, 16m, 17m, 15m
16m front, 17m, 18m and 15m at back, just showing how much bigger in the middle the Naish is and apparently only a 15m
same as above
same again as above
cheers for now,
Robbie