If you are using a slightly thickened epoxy you don't need any pressure at all on the joint area. The weight is just to keep the plywood from sliding around on your worksurface, and to keep the joint area closed up. Otherwise, any tendency of the plywood to bend might cause a small gap in the joint area.

Thicker plywood (and solid lumber) needs more force to bend than thinner stock. If you are using 3 mm ply it takes less weight than if you are using 4 mm or 1/4 inch plywood. For a foot-long joint in 3mm plywood, a couple of volumes from the encyclopedia work fine. If there is a distinct curl in the panel, then you need to flatten it. Add a couple of telephone directories to the other books. Overall, about 10-15 pounds is fine. If you are doing a full 4-foot-wide panel of plywood you'll need a bit more weight, and you'll need to distribute it over the wider area.

Other glues are not as simple or forgiving. Unthickened epoxy works fine, but it has less ability to fill gaps, so aligning the parts is more critical, and a bit more pressure helps hold thing close together.

On the other end, 'Gorilla' glue and its imitators have very good gap filling abilities. Ironically, this works against us with scarf joints on plywood, while it helps a bit with scarf joints in dimensional lumber. These glues react with moisture to cure, and, as they cure they also expand. That's great for filling in rough edges on scarfs in thick wood, but the modest pressure exerted by the expansion of the glue will push apart the edges of a scarf joint in lightweight plywood. Here is where you want to cover the joint with wax paper, then lay on a 2x4 to distribute the weight, and pile on car batteries, sacks of cement, or old bowling balls. The added weight helps to keep the glue joint neat. It is not needed for bond strength. If you apply the glue evenly, but thinly to just one side (and spritz a little water on the other piece to provide moisture for the reaction) the pressure will not cause the glue to squeeze out.