What we do for this is to have a custom property within the part model. We have 2 sketches that give us the overall stock size of a part for filling in the size field in the assy BOM. You would then have to create the link within the BOM to link it to the custom property within the part model. If you double pick on the top row of the column in the BOM it will allow you to change the column type to be a custom propery. You then choose the custom property you want to use for that column.