We once had a neighbor who bought a great sofa and then got it home only to realize that it wouldn't fit up the stairs to her apartment. It sat outside for days until she was able to give it away.
It takes just a few minutes of browsing Craigslist to realize that she's not the only one who had difficulty figuring out whether furniture would fit in her home before buying it. In fact, we recently brought a small table home and then, too late, remembered our friend's advice about marking out the dimensions on the floor with painter's tape.
Here are some measuring tips from FurnitureFind so you don't make the same mistake.
FYI - Room and Board has a service for $25 where they will come and measure your stairs/doors to make sure you can fit the couch/table/bed into your place.
You can give them a long list of pieces, and they let you know which pieces will fit.
Old newspapers are good for laying out furniture dimensions instead of painter's tape