Sump & filter socks - 4 or 7 "

[MitchM]

It depends on how often and how easy it will be for you to swap the socks for clean ones. How dirty is your system going to be?
I'm running a 7" sock on my 600g and a 4" sock on my 150g. I need to change them both every few days. I would prefer to have 7" socks on both.
Fewer changes. That's the hassle part.
One tip I can suggest for changing the socks is to allow an area where you can hang the dirty sock to let it drain for a day or two. It's a lot less messy to move a dry sock to the washing machine than a wet one.
I use a bungie cord and hang it over the sump until it's dry.