I just got my C.5 Boardriding MauiClouds a few days ago. The sizes are 3.5, 5.5, 8.5 and 13.5m. That is a big jump from 8.5m to 13.5m, with no size in between. (I had 9, 12 and 14.5m C2 models before.)
I did quick back to back sessions hydrofoiling with the 8.5 and 13.5m kites.
First session was on the 8.5m from 420pm to 435pm and it averaged 12.5mph (11 knots). It was a comfortable wind for that kite, easy to water start and easy to maintain flight.
I switched to the 13.5 from 435pm to 4:55pm. The wind started with the same 12.5mph average but increased during the session with a peak wind of about 18mph (15.5 knots). I was still comfortable throughout the session....just parked the kite high when the wind was 18 mph. I like the feeling that I could really move, even loop the kite when foiling...so if the wind persisted in the 15 to 18mph range I would have moved back down to the 8.5m kite (and to the 5.5m if averaged 16 mph or more).
Anyway, plenty of range overlap in these kites. I wouldn't want or need a size between the 8.5 and 13.5m...at least based on these brief sessions. I could have happily spent the afternoon foiling with either kite. (Awaiting water pusher board sessions...if I can get myself off the foil).