Sure, I do the EWR-LAX on UA route about once a month. Here are the flights that use the 757 and the the A320.
Flt# Dep time
85 12:20 pm
87 5:40 pm
Thats it, just those two flights, the rest are on the A320. I know that the 757 is larger but the A320 is actually nicer inside, it is a newer aircraft. You can not really tell a size difference in the cabin unless you are in the very back and staring up to the front. I suggest you go for a 320 flight. Their tv system is neat in that plane to. Plus, it's quiter!