At your size, 5.5-6.5 is probably a good choice if you have at least 12-14 knots of wind and
you are still in the learning stages.
The GO 151 should work nicely with a rig that size, and as soon as you are ready you can upgrade
to a 7.5 and probably use the same mast and boom.
Try to spend a little more on the mast, as a light mast really makes a significant difference in over
rig weight and feel.
100% carbon (or at a minimum 75%) will weigh significantly less.
They are a little more fragile, but I have not found that beginners are hard on masts.
Most mast damage occurs "off the water" so get a good mast bag, and treat the mast gently
and it will last almost a lifetime.