by winds, do u mean wings? some tuna guns, like the wongs, dont use wings, they used internal ballasting to counter the recoil issue.
when it comes to distance from handle to butt, well, extend your hand to its natural shooting position, some people extend their hand fully before shooting, if your like that, your handle can be further forward, if you keep your elbow bent, your handle will have to be closer to the back of the gun. what trigger mech will you be using? what size spears? how many bands??
i think that small wings (maybe 1 and a half inches wide) would be the best option, you can put lead ballast in these, instead of in the stock of the gun, and if shaped properly (not the big bulky wings i see all over the place) then they will actually assist a bit in helping your gun turn through the water.
ideally, you'd want a 4 to 6 band, 65 inch or thereabouts gun, probably enclosed track, which shoots with low-no recoil, as for spears, well, tuna seem to bend up whatever you stick in them, so a 5/16th with a sliptip rig should do the trick, some people go with bigger spears, but they are more expensive and the tuna still bend them up
u got a lotta tuna in your area?