I wanted to make a very basic cabinet for my bathroom. It basically required two long planks, yet the price of these two planks was more than buying a ready made cabinet from ikea.
Smaller pieces will typically cost less at the lumber yard, and it also makes it easier to work with salvaged wood -- you no longer need to find big pieces to salvage, you can salvage a lot of little pieces and build them up.
Also note that salvage isn't just dumpster diving -- at auctions and sales you can find large things people made for particular purposes that no one wants, and break them down for raw materials. I've picked up a couple hundred dollars of dimensional lumber for a $5 bid before because it was in the form of a 9 foot tall viewing platform for refereeing volleyball games.