Jump from a 5.5 to a 6.0 is almost negligible. If you keep your 5.5 and want just one more sail, go for around 7.0. Better yet, you can plan a future quiver like this: 5.5 - 6.5 - 8.0. Even 122 will take an 8 no problem (but it's probably too big for your current stage).
Yes you can use small sails on big/wide boards when you're learning, but when you progress, you'll want a smaller board for any sail smaller than 6.5.
Uphauling: with some practice you'll uphaul 122 in zero wind even in bumpy water. A mate of mine is uphauling 111, he's an intermediate @ 80-85KG. As Ken says 122 is better for years to come. Maybe you can buy second hand Carve 133 (or similar volume bic, etc) for 2 reasons:
1. you need to become comfortable in the footstraps, not to struggle to use them
2. you'll probably catapult a lot at your stage (we all did), so a brand new Futura could be ruined soon (but then again, use the nose protector)