No, the line size should not be decreased. I'm sorry you have the free line; its a shame not to use it. Check with your gas company and I'm sure they will tell you the same.
You know, in Kentucky all LP gas installations are required to be handled by a licensed, highly insured installer.
Caution: LP gas is very dangerous, in that it does not dissipate in the air like other gasses. It is very heavy and will settle in a corner and wait to be ingnited. Whole houses have been blown away by it and there is a danger of death.