Hi there, depends a lot on the skill level? I'm 76kg and use the 65L Sky Style, perfect for all-round use, even with 6.0. So perhaps a 4'11"/75L for your 84kg.
But if you want a real sinker, to push under the water, then the 4'5" with 45L is probably the better one? It's what a lot of our team riders use for freestyle and waves, from 60kg to Wesley at over 100kg...