I get asked this question several times a month, and see it on message boards quite often, so I might as well address this at least once. There is no single correct answer other than "wear the one you will be comfortable in" as people come in all shapes, sizes and cold tolerance. In general though, you can typically say the bigger you are the less you will need.
Here in Kona the water temperature typically runs from the 74/75 degree range during the late winter to the 81/82 degree range in the very late summer early fall, with a swing a degree or two above or below those numbers every couple of years.
During the warmer seasons, a 3 mil full wetsuit will do most scuba divers just fine, with larger people doing fine in a shorty or nothing if cold tolerant. Snorkelers will often need nothing, although we always throw in a shorty for the night manta snorkel just ot help for warmth and buoyancy.
During the cooler seasons some divers may need more thermal protection. If you are my size, you probably can get away with a 3 mil or less even during the cooler season unless you dive a lot - the more you dive the cooler everything seems, many full time divemasters here wear 5-7 mil suits year round. If you are slender, a hood or a shorty in addition to the full suit may be a welcome addition. You may find that the suit you are using does you just fine for a couple days, then you start feeling cooler... ask to borrow a shorty or go to one of the local shops and pick up a hood/beanie, you'll be happy you did. Slender/petite women and kids really do well in a 5 mil in the cooler months. Snorkelers may appreciate a shorty in the cooler season, and many shops do rent them for 5-7 bucks a day (typically most shops charge for 3-4 days if you are keeping it for a week) over the counter for people who are wanting to rent one to use. Few shops have thicker than 3 mill full suits available for rent or use on their boats, but most will be happy to layer you if you need it.
It's tough to overdo it. If in doubt, and if you own wetsuits of varying thicknesses, go a mil or two too thick and prepare not to zip up if you feel warm. On shore diving, this may be a different case though, as you don't want to overheat just getting to the water, I'm sort of talking boat diving on this post.
Above is a Praying Mantis we had at the boat wash one day.