To optimise your designs for different screen sizes, you can set styles in different breakpoints. You can do this by selecting the current breakpoint name on the top right of the canvas, and selecting a different breakpoint.
Within a certain breakpoint, you can preview what your design looks like for different screen sizes by dragging the handler on the right of the canvas.
Note that breakpoints in Handoff work from the bottom-up. This means that styles set at a smaller breakpoint will also apply to larger breakpoints.