There really isn't any "standard" to go by in determining wire size for basket settings. I think it really boils down to what you or your client like. Big. bold wire can look very nice with larger stones, as can fine wire, such as 20 gauge. A lot also depends on the intricacy desired in the end. For something simple and utilitarian like a plain double gallery with four prongs, I would probably use flat wire for the upper gallery, say 1.5 to 3mm wide by 1mm thick and probably square or flat for the lower gallery,1.5mm, maybe? You can also pierce the lower gallery from sheet. I often taper the prongs from top to bottom a little. It enhances the hand-fabricated look dramatically, especially with heavier wire.
For a more delicate and intricate look, I go with much smaller wire, sometimes using twinned or even tripled prongs. They can look very nice with a heavier gallery structure. The lighter wire look is also enhanced if the lower gallery is smaller than a standard "store-bought" setting generally is, with the prongs gently curved to follow the pavilion (bottom) of the stone. Don't get it too small though. With a large stone, the pendant will not want to lay flat and will have a tendency to roll from side to side if it doesn't have a relatively flat back.
You didn't specify the shapes of the stones or the intended use, but at those sizes I assume a pendant. For an emerald cut or other squarish shapes, square or flat wire for the prongs can be a nice touch. For rounded shapes I'd use round wire prongs. Try not to just overlap the joints, try instead to notch and fit them together like a log-built house. The heavier the wire you choose, the more important this is to a professional looking piece. I use a round bottom graver of the appropriate size to cut the grooves. Round "rat tail" files also work well but it's harder to keep the width of the groove constant on wider stock. Burs tend to wander and slip.
In any case, the wire you decide to use will look much heavier when the piece is finished than it does when you're just holding it next to the stone to gauge the size of a finished prong. A single piece of 18 gauge wire looks pretty small next to a 10mm round stone, but a double gallery setting for that stone made from 18 gauge actually looks a little heavy. when it's finished.
The opal pendant below was fabricated with 19, 20 and 23 gauge wire (all customer's stones). The two rings show tapered round and flat wire prongs. The round prongs on the oval center are 20 gauge and the sapphire side stone settings are 22 gauge. The flat prongs on the radiant center are roughly 1.2mm by 0.9mm, tapered to just over half of that. The aquamarine sides are set with 22 gauge round wire..The bottom galleries on both rings were pierced from 18 gauge sheet, the upper center stone galleries were fabbed from flat wire, the side stone settings were cut from 16 gauge sheet. The shanks were both forged from approximately 4 X 4 square. All were started from ingots; forged, rolled and drawn into shape.
Hope you find this helpful. Post some pictures please!