I also haven't found a way to resize the fixed columns, but you can slide around the "n:n columns" module to resize the columns manually there. The problem with that one is that I haven't found a way to change it from 5-columns to 2-columns AND behind the scenes the math of the columns is missing 1% (it all adds up to 99% once you slide the columns).
From a technical standpoint, I think the new editor is pretty half-baked. There's a lot of neat bells-and-whistles to it, and I'm sure it'll seem easier to drag-and-drop things at first but I think there's still some kinks to work out with this before I'd think about putting it into production. I've noticed some major errors in the code with the fragments and some fairly obvious oversights (like resizing columns) that make me skeptical to deploy this as a professional-level tool.
I think things like the padding / image-size issue you brought up are going to be headaches for marketers who don't understand the underlying principles of email design and development and even worse are going to present obstacles to troubleshooting in the future for anyone who does actually understand that stuff. In my opinion, this tool is a miss in terms of professional-grade email and behaves more like an entry-level email tool for small businesses.
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