How can I change the size of unique values in ArcMap 10.7

I downloaded fractional landcover data from the copernicus website (https://lcviewer.vito.be/download). I wanted to add this data to a basemap in ArcMap 10.7 but I cannot change the size of the symbols. How can I make the symbols larger to improve visibility? I tried to change its size using the symbology map > Unique Values > Symbol but using this I can only change the symbols color. Is there something wrong with the data itself? Can anyone point me in the right direction?

basemap with landcover data as a tif file