The box size needs to be 50% bigger than the item being cast. 25% from all extremes of the pattern.
Only oil will do. I use linseed as I can get it cheep.
If I am using gas, the regulator is full bore, start the flame gentle, letting the heat work the way through the crucible.
Raise the heat up, the copper will soon start to go soft.
When the metal is fluid, using a pre warmed spoon, remove the dross from the top. pour straight away.