everyone's different but personally I find my current board (i recently bought a tabou 3S 97 litre) usable in up to 25 to 30-ish knots with 4.5 to 5.5 metre sail and 28cm fin, and i fluctuate between about 65-70 kg in weight (we had a 25-30 knot squall on thursday and i could still loop and attempt vulcan with it). With enough practice you would probably handle anything, but generally east coast doesn't get much above 20 (on goldcoast anyway), so I'd say 105 would be pretty good if not still just slightly on the large size?

However, the 3S doesn't feel like a big board considering it is nearly 100 litres. I'm not sure if all modern 100 litre boards feel like the 3S, as this is about the biggest board I have ever had and can't really compare except for riding some friends large boards which did feel pretty big.
I find a steady 15knots with a 5.5 RAF sail on a 90-ish litre board gets me planing no problems on relatively flat water and 17 knots enough to do tricks, in fact 13 knots was ok for planning when i weighed 65kg, but now i'm 70kg
i must be building muscle