Hi Tom, welcome to ScubaBoard.
Regarding wing size, rjack321 and Dubious have already taken a crack at explaining this. Sometimes hearing the same thing in different words can be helpful, so I'll throw in my 2 cents in the bucket.
Your wing needs to be large enough to achieve two things, and two things only. (1) To compensate for the maximum difference in your buoyancy throughout the dive, (2) Be able to float on the surface in case you want to remove it in the water, or maybe toss it into the water and jump in after it.
(2) is easy -- like rjack says, you need a wing whose lift exceeds everything negative on your rig. That would be the backplate, tank, any attached lead, and conceivably gear (like if you have a super heavy can light). A typical steel backplate is -5 lbs or so, full typical steel tank -10 lbs or so. I also like to put 4 lbs of lead in trim pockets. So for (2), I need about a 20-lb wing. If you hate heavy weight belts / harnesses and want to stick it all on the rig, OK, fine. A 30-lb wing supports roughly 15 pounds of lead, a 40-lb could do around 25 lbs of lead, and so on.
(1) is the one that people often are confused by. It has absolutely zero to do with your body's density, the amount of lead you're carrying, the material of your tank, the material of your backplate, or anything else. The big ones that do matter are the volume of gas in the tank, and the volume of the gas in the materials of your suit.
For tanks: At the beginning of your dive, your tank is full, so an HP100 contains 100 cubic feet of air, which weighs about 8 pounds. . Suppose you were to completely empty the tank during your dive (not a great idea of course, but suppose). At the end of the dive, the tank will therefore be 8 pounds lighter, so your wing needs at least 8 pounds of lift to compensate for this difference.
Further reading on tanks:
Here is a chart of the weight of air,
here is a chart the buoyancy of many common tanks.
For exposure suit: At the surface, my thick wetsuit needs roughly 16 lbs of lead to sink it, so we can say that it is 16 lbs positive. As I descend, the gas trapped in the materials of the suit compresses from the pressure. If I were to go super super deep, that buoyancy would go close to zero. So my wing would need an additional 16 lbs of lift to compensate for exposure gear. In the worst-case scenario in my wetsuit, I therefore need a minimum of 24 lbs of lift. In fact, my I have a 32-lb wing, which works just fine. I also dove it in warm water earlier this year, and it worked great.
The material in my trilam drysuit is pretty much neutral, so I could probably get by with a smaller wing. Since you have a neoprene drysuit, the materials will be somewhat positive, but certainly less than a thick wetsuit. So again, the 30-lb wing range is probably just fine, and is a good rule of thumb for all single-tank diving. In warm water where you typically have smaller tanks and tiny or no exposure gear, 15-20 lbs is common. 40-pound wings and larger would make sense if you either had a super duper thick wetsuit (like 2 layers of 7mm, thick gloves/boots/hood), or if you had multiple tanks (adding another HP100 would add another 8 lbs of gas to compensate for), or as mentioned above, if you just want to put a s***load of lead on the rig.
All of the above only works if you are "properly weighted." Which of course, different folks have different views on. What I mean by it is, you are neutrally buoyant near the surface with an empty tank. Because that is the most buoyant you'll ever be, if you are neutral there, then you know that no matter your depth or amount of gas remaining in the tank, you will be able to achieve neutral buoyancy. If you are positive near the surface with an empty tank and empty wing, then you need more lead. If you are negative near the surface with an empty tank, then it is safe to remove some lead. 40 pounds is a LOT of lead, I suggest that you do a weight check at the end of a dive (so that your tank will be near-empty). Just completely dump your BC/wing, and see if you are able to hold a shallow stop. If you sink, give some lead to your buddy or set it on the ground. Keep removing lead until you're neutral. Now you have the perfect amount of weight. As one point of reference, I'm 5'11, 165lbs, dive in 50 degree water, and have been using about 20 lbs of lead in my drysuit / SS backplate / steel single tank. I intend to try a little bit less lead next dive, and expect I'll end up with no more than 16 lbs when I have it dialed in.
Best of luck, hope this helps!
