Quick step by step (I hope it's clear):
1) start the game and change the settings to windowed mode.
2) close the game.
3) right click the .exe file, located in the installation directory (it's named TT_v.0.7.1a.exe or something)
4) click create shortcut
5) right click the newly created shortcut -> select properties
6) behind "target", it should state the location of the exe file. so something like: ...\Taffy Tales\TT_v.0.7.1a.exe
7) behind that, add the lines: -screen-width 1920 -screen-height 1080
The 1920 and 1080 can be anything you want. Though it's probably best if you stick to a 16:9 ratio. This site lists most useful ones:
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It should look something like this:
...\Taffy Tales\TT_v.0.7.1a.exe -screen-width 2176 -screen-height 1224
At least, that's the resolution I'm using right now.
[edit]
Oh yea, and don't forget to start the game from the shortcut you just created.