Rider: 90 kg, intermediate
Style: wake
board: 138 DLX w/ boots
Weather: Melbourne

Can anyone give me some assistance here, was out tonight in St Kilda fairly powered in 20 knots on the Zu's 11m (2012) Razor, however not sure if the 13m will be a better choice for the sub 20 knot winds with boots. Already own a 9m in the same series if that helps.

at 90kg i would say the 13 is a smart move.
i was on a 13 here in perth for 15 - 18 knot days at 88kg which has reduced to 77kg and this summer i find the 13 too much, but bear in mind WA wind packs more drive behind it as i've noticed when kiting here compared to kiting on goldy, brisbane etc (not so sure about NSW but i guess it'll be the same). so To be honest do yourself the favor and get the extra grunt out of the 13m.

Prior to my 13 9 combo the Best GPS I had a 11 9 Razor combo riding a 143 watson and for the most part i was always wishing i had a 13 instead of the 11.

I'd imagine melbs wind would have similar density/drive to Perth being in southern lats/cooler temps (?)

Jees charl, 11kg.. thats quite alot of weight off!