What size is the nut on the line that the fuel filter screws into? This is a 2003 leSabre

I'm trying to get a 16mm flange wrench and it doesn't want to fit on maybe because of layers of rust.

Not sure of the size, I am sure somebody will be able to provided you with that info soon, I will say once you know the size, spray that nut with some penetrating oil like PB Blaster, and let it soak for a few hours, then spray it again, and wait for a few minutes before trying to break the nut free.

I don't remember what it is either, but spray that connection now and keep spraying it.

16 sounds right, sometimes you need to use a file or wire brush to get to good metal. sometimes you need to use a good pair if vise grips to break it loose, aside from the mentioned rust penetrantes

I use a 5/8 line wrench....works just for for me