Ideally, your cast bullets will be sized to match the bore of your rifle, or .001" to .002" larger (there is some leeway, personal preference and variations in rifles/loads, but that is the rule of thumb). You'll want to slug your bore and make an accurate measurement. If you do some searching online, you'll find how to do that, if you're not familiar with the procedure. The Lee sizing dies can also be honed out fairly easily; that's what I did to get a .458 die. That's also online. Keep in mind that variations in mold production and alloys cause variations in the size of the bullets produced by the molds. You'll want to do some casting and measuring there too.
CC Griff