Look at the "Units" property for figures and for axes.
Note, though, that the axes Units property does not affect the interpretation of coordinates such as for plot(): plot() always uses "data" units. If you want "data" units to have physical meaning, then you need to use the "axis" call, or set axes XLim and YLim properties carefully. See also axes InnerPosition and OuterPosition properties.
Note: if you are trying to have lines drawn on the screen be a certain physical length (e.g., you want the user to be able to put a ruler on the screen and have a line come out the same length as it would on the ruler) then that is difficult to do properly with accuracy, and simply cannot be done accurately on some displays.
