I'm not a very big fan of saurii and instead run a southlands inspired list, so even if the skrox ain't the optimal choice (fluff >> cheese anyhow) I often need to run at least one larger unit of em. I've read a lot about tactics etc here on the board, but I've never ever heard anyone running a 17 skink 2 krox unit in a 5x5 formation. Is there any reason not to that I haven't thought of? As I see it you only run large skrox units for the static CR, the one extra attack the 6x4 unit gives is therefore not really that great. The skinks are expected to die anyway, so why not run the unit 5x5 and have some spare bullet catchers instead? The only real advantage as I see it is if you are attacked in the 'wrong' flank and 'lose' two attacks. But then some of the attackers have to target the krox, so it's not THAT bad I guess..? As I said before, I prefer fluff over cheese, but I'd only run 6x4 over 5x5 for pure aesthetic reasons. Do you know any other ones?
Well, a 6 wide unit doesn't maneuver as well. Given the speed of the skrox blocks you may decide you want them trying to flank things, so the extra bit of maneuverability could work out well. If I ever did decide to go for a big unit I'd keep it 5x5 despite the aesthetics.