I am using axure RP 30 day free trial and I have a startup idea. I need to make the website really quickly. But when I at the wireframe stage I encountered with a problem.Problem is, According to which page size (width) I need to design my website? My screen resolution is 1920x1080 but I don't know how to design for all screen sizes? But one more problem: Some people say use 960grid system but when I use 960 grid there is no screen resolution like 960x480 960x350 etc. Please help me I hope I explain well my problem thanks. Sorry for my bad english

Edit: I mean when I design the page for 960 grid Does that design fit bigger screen resolutions for example 1920x1080? Or Do I need to use 1200 grid system or 980 etc to fit the design bigger pages than 1024x780 please help me I confused too muhch