EZ Pop basic can upload almost anything that you can set manually in Admin. I don't see a spot for size: do you have custom attributes? Anyway, a good way to find out what cols you need is to set something manually, then download the EP file to see what data cols you have to work with. I'm sorry, I don't know anything about attributes.
I'd suggest adding a tag to the end of your descriptions. In Excel, you can add a new line in a cell by hitting Alt-Enter. If you have your desc in column b and the size in column d, you can concatenate the desc with the following formula:
=B2 & CHAR(10) & "<b>Size:</b>" & D2
This would give you the desc from col B, then a new line, then html for bold, the word size, and the size data from column d (all in row 2).
After you make this formula in all of the rows you need, copy it and use edit/paste special / values to replace the formula with text. When you upload it will be fine. You can add any HTML that will work in a desc: maybe an HR? I've never tried that. But you can certainly format it differently enough so that it stands out at the end of your desc.
Sorry if this was too much info! Hope this helps.