MATLAB: How to save a resized figure, with extended name ‘jpg’

figure resizeMATLAB

I want to save a figure with specified size (for example 1000*400 pixels), rather than the default size. I have resize the figure with property, 'position’,like this:
plot(rand(3,1)); set(gcf,'position',[0 0 1000 400])
When I use "saveas" to save the figure into a file with extended name 'jpg', the figure size is default size, not the specified size. How can I save a figure into JPG (or png, tif, emf) with speicfied size?
Thanks in advance.