You guys are nice.
As for the question about the size of holes, it does depend on what’s going through those holes. In any case, the holes should be just big enough to allow through both the stitching material (thread or lacing) and whatever’s pushing it through (a needle with an eye, a Speedy Stitcher, etc.).
This, BTW, is a Speedy Stitcher. The thread goes inside it, so you don’t have to push doubled-up thread through your piece, and can interlock additional thread on the bottom side of the piece.
If you want to use thread, the holes should be just big enough for a needle and the doubled up thread to go through them. If you use leather lacing, you need to base hole size on both the size of the lacing and the insertion method.
Whatever the case, you should do what you can to make the holes dots instead of slits (which are more likely to rip), and keep the holes as far apart as you aesthetically and structurally can, unless your object is getting very little wear. The smaller the stitches, the more holes you’re putting in your leather, and the weaker and more likely the leather is to rip along the line of stitches (which is a nonrepairable failure). That said, I sew leather decorative elements onto leather all the time just using my Singer Heavy Duty sewing machine, with stitches that are quite tight because I know the objects won’t be getting hard wear. Things like belts and wallets need to have well spaced stitching, though.