Sawdust fluxes (cleans) the unwanted stuff from the alloy. It also reduces the oxides. Wax, or paraffin will reduce your oxides. You use sawdust and/or wax in the smelting pot. Once clean you don't need to clean it anymore. You just need to reduce your oxides in the casting pot so use wax.
Small or large sawdust will work equally well but the fine sawdust will char quicker and the carbon will act as a sponge to remove the unwanted stuff from your alloy source. Let whatever your use, fine or large, chars or burns to carbon and stir and scrape the pot . Do this till your alloy is clean. There is no set number of times you do this. When it's clean, you pour ingots. Use those clean ingots in your casting pot and only use wax or paraffin to reduce your oxides.