As far as swimming room goes, the longer the better
You mentioned 6' and 7'
First thing that comes to mind is trying to find a ready to use light fixture for a 7' tank
If I was in your shoes and had the room, I'd go 8' and use 4' fixtures, or stick with a 6 footer
Unless you're planning on using LED pendants, then just go big
Having all the equipment in another room is so nice for noise and access