Typically mains cabling carrying larger currents appear to be specified by the area ( sqmm). Some speaker wires too. But the most widely used method of specifying wire is by the guage . SWG for metric units and AWG is the older ( in inches and still used) American system. If you don't know the physical dimensions ( diameter or sqmm section ) of any guage , you will have to look up the charts available on the Net. If you use certain sizes often , the physical dimensions will be easy to remember.
Needless to say , the key factor here is the
cross sectional area which determines the current carrying capacity and resistance per meter/foot /km what ever is specified. This is followed by lots of other parameters some of which might be important depending on the application. Like voltage ratings ( due to insulation ) , capacitance , insulation types , conductor material etc.
Often you can only decide what is good for you after you try it out yourself in your system. Cables that you can't access or are too expensive should be ignored. They are not going to help you. Often simple solutions work out very well. Do some experiments !
One example I've mentioned earlier is the use of enamelled wire of suitable size used in multistrand mode designed to do what you want.
In our case we used it in a "bi-WIRED" system and it brought up the snap of kettle drums/percussion apart from other things! Speaker cables must be kept as short as possible. The ideal system should have the power amp right next to the speaker with short cables and inputs should preferably be balanced XLR connectors. This would give you very wide bandwidth , good RF and hum rejection and minimal effects from the speaker cables.
I saw someone suggesting loading the signal level cables with 100 ohm resistors to achieve a higher roll off frequency. This is fraught with dangers. Most consumer preamps do not work very well with very low loads like 100 ohms. Distortion goes up as the load decreases and signals might clip much earlier than the unit was designed for. You could design preamps to work well at 100 ohms or even lower but that's not what most circuits are designed for. Don't be afraid to experiment ! Your cheapest skimpiest solution might sound as good or better than expensive solutions in your system. You never know till you try it out.
Remember the speaker cable tests from long ago where they used several expensive cables and also a straightened aluminum wire from a ( clothes) hanger ! It was a blind test with the listeners not knowing what cable was being used or even what the cables were ! The aluminum wire topped the list with the 'best' on the listening test ! It doesn't prove or disprove anything. It just shows you that you need to test your choice in "your system and be evaluated by your own ears " ! You don't have to be an audiophile with 'golden' ears ! You just want to get good sound as "your" ears tell you !
There is never any single answer for the question , "which wire is best for my system". I just did a simple Google search and lots of interesting links came up. I don't have time to go through them all, neither would I do it. There just isn't enough time. You could however look at some if you have the time. I must say I spend more time nowadays searching for good music that's recorded well. But here is the Google link I searched for....
https://www.google.com/search?clien...num+wire+from+a+hanger+in+speaker+cable+tests