In most cases a cable length of around 50' with 16 gauge wire will net just under a 1db loss in level. This is negligible and you probably won't even notice. Now if your running the same setup with a 100' cable of the same gauge, then you will have about a 1.5db loss in level. This may be enough for you to notice? To me anything higher than a 1db loss is not good and wasting power. Not a super big deal to loose 1db, but if you have a 200' run you almost loose 3db which is significant! The basic rule I follow is to use as large a gauge wire ( 12 gauge is fine for mains and real short runs on subs and 10 gauge is better for subs if you can't keep a real short run ) and as short a cable as practical.
The common low end wire gauge is 16 and a good mid level cable will have 12 gauge. I prefer the 12 gauge as a standard and shoot for 10 gauge on subs or very short ( less than 10' ) runs to them. I will commonly place the amps for the subs directly behind them using as short as a 3' cable. I am perhaps a little coco about keeping power available to the subs? To me the subs are usually neglected and underpowered ( to save money ) and I like to get as many available watts as I can to them if the need arises. I seem to end up working for a few companies that just don't design systems with performance in mind. They just seem to want to put a system out there that will run and sound ok doing it. My rig is deployed to run like a well tuned Porsche and I never worry about available power, headroom and system limitations. I spec my rig well and know what it will and won't do and know that when I book a gig for it, it will outperform the clients expectations. I also never worry about component abuse, as I know that in most cases I am running my system at a cruise speed for it.