I'll throw a couple more variables in here...
If you're looking for grass finished beef, the cow will be significantly smaller. Corn finishing is a way to make a bigger cow. (There are arguments both ways as to which is better here.)
We're a family of 2 and eat somewhat low carb/high fat -- so we eat a bit of meat. A quarter (grass finished) wasn't as much as we thought it would be. Where we got it we also did not get quite the selection when buying a quarter. In other words: for a quarter beef, there was a fixed cut sheet. For a half, you could have it cut any way you pleased.
Your mileage may vary, depending on how much beef you eat, etc.