Size() is a table calculation function similiar to index(), first() and last()
Here is an example workbook that I made up awhile back that may be helpful in understanding these functions. Index() First() & Rank()
Size is relative to the number of marks.. I have used it both as a set field value and as a calculated field - just depends on how the view will change when users are interacting with it.
Does that help?
Thank you Patrick!