If you are using resistors from, say, the
rcl libraries, the resistor part has multiple footprints already of all different shapes and size.
In this case all you need to do is right click on the part (either in layout or in the schematic), and click the option named
package. This will bring up a window of all the available packages to choose from.
The context menu looks something like this:
And this is the window that appears:
So in my case I was using an 0402, but can change it to one of many other sizes. Simply select the one you want.