I don't use muro but it doesn't look like muro.deviantart lets you resize an image or change the aspect ratio if you start with a blank canvas, so my understanding is you'd have to import an image at the correct size (from sta.sh? I don't use that either). And the beta version has crop and resize? Sorry, I just don't use these tools and don't really know about them.
BUT, in terms of laying out comics, and why things go where they do, that's based on professional illustration practices, and I wrote about that on my Tumblr, if you'd like to read it. (This is a bit technical, but the size and layout of my comics personally are based on how they'd print in a book.)
In terms of general comic layout, and how to size panels, etc. I would recommend this article:
If covers some things you don't need to pay attention to (clipping masks and vector stuff), but pay attention to the part about types of panels and camera angles. In general, too, a big panel = long moment. Small panel = short moment.