When creating a new graphic you have to select the DIY template if you want to set a custom size (our templates are of course a set size). When you use the DIY setting you’ll get a width and height slider you can use to set the size (or just enter the number below each slider).

For images you have already created, go to “My Saved Graphics”, locate the banner and click on the resize button. The use the sliders to set the new size and click resize.