First of all, if you want to make the mold that you pictured, you cannot do that with a drill press. It has to be done on a milling machine with a round end mill. The reason being is you have an elongated slot and you must cut this out, which you cannot do with a drill press. Also you have to make sure that the cavities are aligned and pinned. If you don't pin them then the mold halves will have to have a hinge.
Wood is not a good choice, as It will start burning from the hot lead, and eventually the cavities will crack and get charred from the heat.
Most molds are cut from aluminum. Reason being, aluminum is easier on the tooling, it cuts fast, and it dissipates heat when you pour lead.
As far as sizes go, unless you know what the mass weight is of the size you want, it will be trial and error. Also depends on the lead properties. Pure lead is heavier than alloyed lead.
This is just a start. There are many other thing involved in doing this. If you insist on doing this, you should also have a technical drawing showing sizes, depth, length and position of the cavities in the mold along with all of the dimensions, so you can duplicate accuracy.
You can make molds out of high temp silicone, but that is short lived as well.
Good Luck