As Tom says, the size of the power pack is dependent on the load, not the amount of track. A load is anything you hook up to the power output and includes engines and accessories. Most power packs are rated for total load, so even though you're only using the track power output to run engines, the amount you use for accessories such as lighting or turnouts, adds to the total load, even though they may be connected to the accessory output.
Again, as Tom says, good wiring practices have a lot to do with good operations and consistent voltage across the length of your tracks, especially with long track runs. I don't want to just repeat what someone else says, I'm just trying to emphasize that these are the two major factors in selecting the right size power pack and how you wire it to your layout.