There didn't seem to be any demo or sample project with this, and the manual lists the assets you can import but not their sizes. 

128x128? 256x256? Are weapon and entity (decoration and enemy) assets a different size than the walls and tiles? 

Or to go another route, is there a sample project I can download to see the proper asset resolutions needed?

I second this

there is not any hard set sizes, the engine will upscale textures if deemed to be too small (atleast from what i believe) but 64x64 is about the smallest id go for wall textures, weapons are just about whatever, and pickups and text already has sample art. you are rather free to use just about any size for anything in the engine