70 cm Mares Cyrano speargun.

Those sound like awfully small spearguns for Florida - isn't the water round Florida particularly clear? Perhaps if you are after fish you've seen holed up in caves/gulleys that might be ok. The size of speargun is more concerned with visibility & how close you can get to the fish you are targeting than your size though(take a look at the size table at sumora, official distributors of Rabitech and Rob Allen). That's easy for me to say because I am quite big & have fairly short guns (90 & 75cm) but, while you might make some allowance for your size/weight, usually it is not the main factor. For example, maybe you'd be better off with, say, a 100cm (give or take 10cm) gun but with less rubber to load e.g. a single 14mm or 16mm band (you can always move onto more/bigger rubber later, e.g. 18mm or 2x14mm, if you found the need; I find a single 16mm powerful enough for all my needs). Loading is largely a matter of technique; a good thick loading pad to protect your chest certainly helps though.