How can I change the size and font of the SALE/NEW/-30% label?

It is not possible to adjust the font and size of the labels separately, unfortunately. 

All settings that can be applied are available in the block settings → Sale/New marks (text/background color). 

You can try to apply changes using the third party code. 

Read more about the html embedding function: https://help.tilda.cc/html. 

Please note that Tilda support does not provide advice on third party code.