thanks guys.

@madman initially it was 1920x1080, and, believe it or not, it seemed to run smoothly with that large of assets. I was having some particular issues in my tests with crashes/stability, though, with that large a screen and that many assets, and rather than wait for all of us to figure it out I made a 1280x720 experiment and the issues went away. I still think full 1080p would be doable, I'm not sure yet what it would take to make it totally stable, but it never seemed like the sort of thing that couldn't be fixed. Most things can be fixed, either by us as creators or the Stencyl folks in charge of the toolset, if effort is put in and the issue is isolated. I stayed with 720p because it looked great to me full screened, upressed with the AA turned off -- it's a tad "pixelly" looking but it suits the game.
If you're going to develop an ambitious title mostly by yourself, you may occasionally need to make a compromise, and I felt more comfortable going forward with 720p when there were so many unknowns and using software that wasn't originally built for what you're doing (as far as I know stencyl was originally conceived more for flash/mobile), especially since it looks quite good full screened. This choice increases stability and makes it easier to deal with memory management since the graphics are smaller and take less memory, which is important in my case since I use a lot of different graphics.
I think most people will be very pleased when they see how it looks on a full screen, it's not ugly or blurry. I run it on my 42 inch TV all the time, and I think it looks really great there.
re. map planning, I lay out the maps on a grid where each rectangle on the grid represents a screensize. Start by envisioning in your head what the game would be like to play, and lay out rooms of the map, on the grid, accordingly. And then start implementing those as individual scenes in Stencyl. It's an organic process, and, in my opinion, once you figure out the basics, making a metroidvania is really not that hard -- at least, it's not any harder than most other types of games (making any game is pretty hard).