If your going to do 12v lighting stuff order automotive wiring (waytekwire.com for example), the resistance is less there and therefore less loss. Unless your running from one end of the house to another then you won't really need to worry about the loss on automotive wire. Using 120v wire for 12v stuff is where you have the most loss, it typically has a higher dc resistance because its designed for a lower impedance (impedance is basically resiatance for AC).
There are also plenty of charts on the internet where you can look up the proper size for length and current.