The Apollo Lunar Module had a total mass of 14,696 KG. Of that, 2358 KG was the propellant for the Ascent stage and 8165 KG was the propellant for the Descent stage. Had there been easily accessible propellants available for refueling on the lunar surface, they wouldn't have needed to carry down that Ascent stage propellant. In turn, the LM itself could've been quite a bit smaller since it wouldn't have needed nearly as much propellant. Or, it could have carried a much greater payload to the surface.

It's unlikely they would've used lunar propellants for the Service Module, so it would've been about as massive as before. Had the LM been made smaller because it didn't need the Ascent stage or it's propellant, then it would've taken less propellant for Lunar Orbit Injection. That could've cut the size of the SM somewhat. Some things would've stayed the same, such as the mass of the Apollo CM and the amount of propellant needed to bring it home from lunar orbit.

Weight gains have a tendency to snowball. Every extra kilogram at the end of a lunar mission means you need many extra kilograms at the start. Still, I doubt we could've cut the Saturn V payload capacity in half and still fly the mission with lunar surface propellants. It'd be an interesting exercise to run some actual numbers to find out for sure.