Title says it all. Since there isn't any ultra sun available near where I live, I decided the next best thing would be to just buy it off the eshop. So what's the size of ultra sun when it's downloaded?
3.6 GB, or 29500 blocks.
Source: https://nintendosoup.com/pokemon-ultra-sun-ultra-moon-file-size-revealed/#:~:text=According to a product listing,region locked on Nintendo 3DS.