By taking notes in the field, you not only get it down while it's fresh in you mind, but you don't run the risk of forgetting to do it or putting it off until later. Memory is at best a tricky way to document birds as the tendency is to remember more than you actually saw. One important thing to remember is to include only those field marks that you actually observe, not what you think should be there or what someone tells you that they see. These are your notes after all, so why confuse the issue by including things that should be there but that you can't detect. Things to look for are numerous in some situations and less so in others, but it's important to record even the most minute details, as they may matter in differentiating one species from another.
Body size is always a handy thing to know.
When birding, this virtually always means a relative size . Actual sizes are always subject to challenge unless you have the bird in hand and measure it. Since the most difficult identifications are often a matter of fractions of an inch, guessing at a birds dimensions are worthless. Shape is also useful in differentiating some birds so include this in your notes, but use terms that will mean something to others. Shaped somewhat like Aunt Martha may mean something to you, but is of little use to anyone else.