I know that electrical resistance will decrease if you increase the cross-sectional area of the wirebut if that's the case, then why aren't wires made larger?? There must be some reason, some con to increasing the 'A'? I haven't touched Physics since high school, so I really don't remember. Thanks.
Cost and space.thicker wires cost more and they would be too heavy for pylons. Silver has a lower resistance than copper but it's not used because it's too dear.
Theoretically - can, but practically u need to check. You consider the the maximum possible measurable length between the two measuring point, as total length of conductor and take twice the cro-sectional area. In practice length is reducing and diameter is increasing so the resistance will reduce. The diameter will be the length here.
There would be no point to using larger diameter conductors. They are kept to a useable minimum, mainly because of cost. If you double the diameter of a wire's cross section, it has 4 x the area, and weight, and therefore cost. Aluminium wires are now often used for further cost reduction and weight savings.
The only exception to the excellent answers that have already been provided is in the case of radio-frequency (R.F.) currents. R.F currents are caused to 'crowd out' to the surface of the conductor. Consequently, the R.F. resistance of a conductor can be many, many times that of its d.c. resistance. There are two classic methods to combat this (undesirable) effect. One is to use Litz wire (look it up on the 'Net); the other is to use wire that is much thicker than what is required for the d.c. current that it passes, but the increased diameter of the wire produces a larger surface area - which is usually silver-plated, to reduce the R.F. resistance even further. Litz wire is usually used in the frequency range 100 kHz to 2 MHz; the silver-plating method is usually used in the frequency range 50 to 1000 MHz. .
The con is the cost of the wire. I know this sounds simplistic but it's the truth. It's an economy issue. the area of highest resistance (all other things being equal) would be the area of lowest diameter. so the lines carrying electricity into a neighborhood tend to be large (but not pure copper), descend to smaller lines (also not pure copper) to feed individual homes, and the gauges of the pure copper wire in the home vary depending on the load (your stove or electric dryer have larger gauge wire). All of that wire was selected so that it could both handle the total load AND balance the cost of extra resistance vs the cost of a better conductor vs. the strength of the material. Copper is a poor choice for high tension power lines because it is too soft. and would break under its own weight. If small wires under heavy load they heat up due to the resistivity. The proper wire size is one that can carry the load without heating up significantly. but it would be foolish to exceed a certain diameter because copper is expensive.