If you mean you want to change the size of an existing figure without changing the size of a plot that is a little more complicated (but still involves the same idea as below, explicitly setting the axes properties).
But if you just want to create one then create the figure of whatever size you want, using the 'Position' property in the call to figure. Then do the same with the axes function, also using 'Position' (and also 'Parent') e.g.
hFig = figure( 'Position',... )
hAxes = axes( hFig, 'Position',... )
Depending if you want the axes to be fixed size or relative to the figure you can also change the 'Units' property of the axes between e.g. 'pixels' and 'normalized'
