Open up the Report Properties either by right clicking on the designer background or selecting it in the Properties window.
If you right clicked and opened the Report Properties window, then click on the Layout tab and set the Page width and Page height properties. Height = 8.5in, Width = 11in. This will cause the report to render in landscape.
If you select the Report from the properties dropdown, then it's called PageSize. Set to the same values mentioned above for landscape.
Be careful with your margins, your report body should not be wider than your report width minus the sum of your left and right margins. So if your left and right margins are 0.5in and your report body's width is 11 inches, you will spill over onto a second page. Your report body's width should be 10in in this scenario.