If I understand you correctly, you just need to get a blank column whith title 'Size' in the report, do you?
When you create a report, click 'Change report format' button, select from the available columns that which has no value - and rename it.
I guess you want to poulate these columns with something as well?
If you wan't to extract data from an attribute of a component, have a look at the in the User Attributes button located on the Reports tab.
All the rows you add here will be available for use in the BOM report.
Please select "Accept As Solution" if this post answers your question. Kudos don't hurt either. ;-)
You will need to add the column to the access database. make SURE to add it to the end of it will cause other problems. The columns MUST stay in order but you can add whatever you want to the end.
then your component need to have the same attribute on it.
a simpler fix might be to use 1 of the existing colums and label it as needed