Gauge is based on average continuous current draw of the device, as well as wire length. So you need to find out the continuous current draw of the light bar and the CB. That might be listed on the device, or the manual/datasheet for the device, or you could be very conservative and use the rating of the fuse.
For the light bar, you would want the ignition-switched power to go to the switch and then to the relay, such that the light bar would turn on with the ignition switch and could then be turned on/off with the push button switch or whatever you want. Anyway, relay power needs to come from ignition-switched power if the light bar switch is a simple one-in, one-out. This may change slightly if the switch is a different type.
Power from the light bar can come directly from the battery. The relay coil is ultimately the only thing that should come from ignition-switched power, so the light bar is always off when the ignition switch is off.
Pretty much the same for the CB, with maybe a few minor tweaks. If the CB has a power switch already built-in, simply adding a relay and an extra switch should be all you need. You would want the switch to choose between constant power (for when everything is off), and ignition-switched power (probably for everyday use). That kind of makes a failsafe for everyday use so you can't forget to turn the CB off and drain the battery overnight.
