Depends. You may get the size before the write completes. You could get the size after the write completes. You shouldn't get an "in-between" number, as the individual write(2) system calls should be atomic; it's just the ordering of when they occur that's up in the air.

Aside from the fact that you probably should be using stat, rather than seeking to EOF and tell presumably, you may get more germane answers if you state why you're doing this and what you think you're finding out.