mobile site is additionally incorporate in responsive site. You have to invest tad bit more energy to make your site responsive.
You say Why? Since now a days a few individuals use mobile , a few individuals use tablet and a few individuals use desktop PC. You have to make your site for each client , easy to use. This why I lean toward responsive outline on versatile site...
But if your site is non-responsive, you just need to make its resolution between 700 to 900 pixel.. (900 pixels definitly works for mobile, Pc or tablet).. remember that put content on left side, so visitor can read easily..!
good luck!