As a rule it is always suggested that you try and design for the screen size that any given dashboard will spend most of its time being displayed on. For example 1280 x 1024. This allows you to maximise the real estate and avoid wrapping text occuring in text boxes, or zooming causing a distortion of aspect ratio on yoiur carefully crafted designs.
The dashboard itself has no formal functionality to restrict your design to a particular size, however one easy trick is to add a shape and set its properties so you have a visual guide of the space you are designing for. To do this:
This rectangle can be used during the design, then removed at the end once you are done.