I learned the target platform lesson the hard way and ended up having to scale a larger project down (playing around with 1136x640 for iPhone 5 and then scaling down to 800 max width for Scirra arcade). Not a big deal for simple projects but could be hard with a more complex game. The best advice in addition to choosing a target platform before hand is to target a specific aspect ratio (see Ashley's "Supporting Multiple Screen Sizes" tutorial). That way the game can scale predictably on a number of different platforms. Good luck and let us know how it goes.
Also Gillis your Greenlight project looks awesome...a thumbs up from me!