Lemur can work at any size, but resolution is set per-project. Use Lemur Editor to set the resolution of your project or to resize an existing project. You can see the resolution of your particular device in the Lemur app settings screen. Use the values displayed there in the Lemur Editor.
The Lemur Editor offers a stretch function. When you change the resolution, you'll be asked if you want to automatically stretch the project. This function has some limitations: some objects have minimum sizes, they can't be resized smaller than that, and some projects use scripting to move and resize objects. The stretch function can not analyze scripts.