Eddie,
I don't know where your info came from but some of it isn't usual at all. All CSA figures are mm squared CSA.
1mm CSA used to be the standard for lighting, with the move to more & more lighting fixtures in the average home this is now more likely to be 1.5mm CSA. 2.5mm CSA is generally used for power ring mains, with multiple ring circuits now being common. Each ring is effectively limited by length of run. 4mm / 6mm / 10mm CSA used generally for spur circuits.The choice in each case is not just dependant on the current rating of the cable but also length of run and voltage drop / length.
The move to miniature circuit breakers has also changed the nominal trip ratings to 6A, 16A, 32A, 40A etc. There are various cable manufacturer's and distributor's web-sites that give current rating details. The actual current that a wire will carry will depend on many factors including the sheathing and is always subject to qualified interpretation.
All this is immaterial of course for model railway use.
As previously stated, 2.5mm CSA for bus runs will be fine for most users. If you need longer runs because the layout is BIG as opposed to LONG then it would be better to stick with 2.5mm and use multiple boosters. If it's more like a garden layout and is just LONG then OK push the CSA up to get better volt-drop characteristics. 2.5mm singles are good for 20-30A, more than enough, the choice is just down to volt-drop and cheapness. (It is almost as cheap as 24/0.2mm /100m from Maplins for instance.)
Droppers of 16/0.2mm, 24/0.2mm, 32/0.2mm or 0.75mm /1mm CSA will be fine for almost everyone. The droppers are generally so short that their CSA can be whatever is most convenient. (i.e. volt-drop is less relevant)
16/0.2mm is equivalent to 0.5mm CSA and is good for 3A. If multiple droppers are fitted to each rail then there should always be two paths for the current, so this allows for 6A. 24/0.2mm is equivalent to 0.75mm CSA and is OK for 6A. So again with good practice you are now able to support 12A.So say you have a short on one length of track will your wire melt or will your booster trip? The way to find out is to put a (2p coin) short on _every_ piece of track all around the layout. This is a fairly standard DCC system check. If your booster trips each time then you know that your CSA and length of run are big and small enough respectively. If a single piece of track (say a separately wired frog?) fails the test then it needs another or heavier dropper, if a whole area of track fails then the bus wires aren't up to scratch. Failure to carry out this check is asking for a meltdown one day.
You don't have to do any of this of course. It's your railway.
Regards
Len