13-inch rMBP: 2 questions

I am probably buying the 13-inch rMBP for school. I have 2 questions.

1. How does Windows 7/8.1 perform?

2. I don't understand the scaled resolutions. I currently have a 21.5 iMac from 2011. How is my usable screen size? What happens if I change the resolution setting?

Thanks! Sorry, english isn't my native language.