If you upload your workshops using the hex editor method in the following guide, ignore the text below.
https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=1627692828

The new workshop square is 119 x 119, and that's what the templates and textures are sized for in this guide, however if you want to avoid black borders they must be uploaded at 150 x 150 pixels. You can either use the templates in this guide and just upscale your final crops to 150 x 150, or use these larger templates I have also created:

• 150 x 150 Template: https://i.imgur.com/taEPWNs.png
  
• 150 x 150 Texture: https://i.imgur.com/5tMcBaZ.png

I will be looking for a method to avoid Steam's automatic downscaling so we can upload crisp native 119 x 119 without it being downscaled with black bars.

You can upload native 119 x 119 without black borders if you use the upload method in the guide above, but it is kind of complex.