My "trunk" (no trunk, it's a wagon) mount battery works just fine with aprox. 25' of #2 cable for the positive lead, and grounded through the rear subframe/body. This might be a problem with a weak battery and a car that you have to crank forever to get it to start, but with the LS motor (starts almost imediately), and Odyssey ER40 battery (1,100 cranking amps) I have no problem at all.
As far as what size wire to use for remote posts you need to consider the total circuit length, how many amps you're trying to move, how long you are going to be moving them, and what other restrictions there are in the circuit. With remote posts for jumping you want them as close to the engine as possible (preferably right off the starter) if there is a good chassis, or engine ground available you don't even a post for the ground. As far as a trickle charger goes l would just install a small plug.