Hi Philip, where are you windsurfing - 5.3 as "lightwind" is cool!!
well you could probably go for a 100, as the wind seems pretty strong where you are. However, for your body weight the 109 is just right i would say! - gives you some extra volume when you need it in slides and stuff...
Spocks and Grubbies:
Probably it's not the board that doesn't want to slide - it might be just some more practise that is missing...
Hard to say without seeing it on a video or in real, but for the Spock it's important you grab the other side of the boom at the same time as you rotate the board, so that you are almost on the other side once you land. also keep your weight on the front foot and extend the back one... you need to force the board to slide backwards, then you will also get it spinning...
Grubby: almost the same thing - try not to jump too high, rotate by lifting the back foot slightly to get the nose into the water and rotate around it. Extend the back foot once you are facing the wind. Don't twist your body, extend the sail hand and bend the mast hand to let the sail help you rotate.
Start the Grubby (jump) out of a slight carve downwind, not just from sailing straight downwind, that will help you to keep on rotating after landing.
Forwards:
It's definitely harder to do them on a 109 liter board - just so much more volume - also the volume prevents you from pushing the tail of the board really down to create that pop when taking off.
If you're landing them on your wave board, just keep on practising - you need to develop the perfect "light wind" technique...
later
Rossi