First thing I need is to have an attribute in Magento admin populated with values. There are 318 products in my store, and about 200 - 250 of them have the attribute called "Size". There are about 50 - 75 unique attribute values that need to be inserted using Magento admin interface. Second thing is to go through each and every product and assign them the correct Size value, according to neatly formatted xls spreadsheet that I will be providing. Sample spreadsheet attached.
When the work is done, every product that has a specified "Size" attribute in the spreadsheet, should have a dropdown attribute called Size, with the correct value selected, according to provided spreadsheet.