I am testing an application and need to test it with different screen resolutions with windows form applications.
What is the common resolution tested for now days?
That depends a great deal on your market/target audience.
I think your safe minimum 1024x768 for desktop apps but don't forget that mini PCs are taking off and they have smaller screens than traditional. If you're gonna run on one of these you need to consider "funkier" resolutions as well.
As far as maximum resolutions... the sky's the limit. Though i think if you look good in 1280x1024 you'll be fine in anything larger too.