The size of the holes is not really important. It´s basically a question of preference. I made a couple of knives (from 440C though) and used 3 mm brass pins or even some brass pipe 7 mm outside diameter and 1 mm wall thickness. I would recommend to use a small size for the brass stock. It tends to add a considerable amount of weight otherwise.
Make sure that the material for the handles is soaked well and any excess of resin is squeezed out. Pockets of resin will not only look ugly they will also weaken the material.It is important to press the resin through the material. If you try to soak it before application you might end up with air pockets. That will ruin your handle. I used this technique to make handles of paper. I works pretty well. If you make the handle right on the tang you can avoid gaps caused by a slightly uneven tang. I also tried to mold a handle with resin filled with graphite. That worked well too and looked really nice. It was too brittle though.
Try to design the shape of your handle and make a model of cardboard first. That way you may ruin the cardboard model and start over with little effort. A slight error in the design can make the knife really uncomfortable to handle. The same applies to the blade. I discarded some of the first of my knife designs and it was worth it.