Accessory Wire?

I use stranded wire for wiring around the layout, it's much easier to route, and not nearly as fragile if you move it a few times.

For most accessories, #22 will be sufficient unless it's a really long run from the power source.

Here's a handy Wire Drop Calculator, use it to calculate the drop based on the current draw of the accessory and the length of the wire run.

I use 18 AWG stranded for everything.   Just easier for me to strip, connect, and work with than the really thin wires, given my poor eyesight.