Not a dumb question at all.
Length and size do matter. Sizing is important in order to deliver a consistent pressure and flow or gallons per minute to each fixture.
Too small of size with high pressure creates too high of velocity which will wear out the tubing and fittings. Too large of pipe size can create areas with too low of movement and lead to stagnant water.
Sizing of water lines is determined by figuring out your total load by number and type of fixture. Knowing which pressure range you have. And distance overall from your water main to the furthest fixture (also each branch to a group of fixtures like a single bathroom can be calculated the same way). You can also calculate friction loss by number of elbows and pipe size, but that's really for larger projects and engineer nerds.
This code reference could help in sizing for your jurisdiction.
UpCodes offers a consolidated resource of construction and building code grouped by jurisdiction