In DS you use the Render settings under GENERAL you can select DEMENSION/CUSTOM. DS renders at 96 dpi and I don't believe there is a way to change that but you could just render at 150x8=1200 and 180x11=1650, So your render size would be 1200 x 1650 or you could just use 96dpi to get a physical print size of 8x11 which would be 768x1056 in the render size settings. But I usually render things at a higher resolution and resize in photoshop which seems to work better.