An update was released today for this game, including windowed mode support and a way to force it to run at a specific resolution, maybe this can help.
To set Launch Options in Steam, right-click on Bastion in your game library, select Properties, then select Set Launch Options.
NOTE: Please be advised that these settings are recommended for advanced users and may adversely affect your game experience.
The parameters are:
-isomovement
Using the default Slinger-style WASD controls, diagonal movements will more closely adhere to the isometric perspective. We would appreciate your feedback about this change via our Steam forum (linked below).
-windowed
Forces the game to start in windowed mode.
-x [value]
Forces the game to a specific X resolution.
-y [value]
Forces the game to a specific Y resolution.
Example: to run the game in 1280x720 (720p) resolution: "-windowed -x 1280 -y 720"