The easiest is just to use UTP cable, with low capacitance and twisted pair.
Well really with capacitance (what the RS485 specification is meant to reduce/counter act), you need to add up all of the capacitance in the chain. If you have 15 slaves all 20 meters apart, that comes down to a total of 300 meters. Plus the connectors, though being made of solid metal, they don't add to much.
with CAT-5 UTP you should be able to get up to 300meters at 250kbps and reduce the baud-rate by half should double your distance.
DMX which uses cable with slightly higher capacitance (but twisted), is transmitted at 250kbps, and is more or less guaranteed to work with chains up to 100 meters (it includes a lot of big XLR plugs) but usually much longer. For more info look at Capacitance Wikipedia and Twisted Pair Wikipedia
It can of course, but if you have the in/outputs of the transceiver near the edge you should not have to be bothered about it. The A & B traces should not be wide of course, they are for 'signal' not power.
Important is that they connected to a stable power source.