You can either use the Stretch command (Edit > Reshape > Stretch) or the pet palette’s Move Node icon to stretch/shrink the Object by a node.
Note that you stretch the A and B parameters of the symbol, not its bounding box. Using parametric 2D scripts, you can easily create objects whose symbol size will not match the A and B parameters. If you make such a symbol, make sure that you define hotspots at the corner of an A*B sized rectangle. These will enable you to stretch that symbol.
When stretching Doors & Windows, you can use reference points outside the host wall. The endpoint of the stretch vector will be projected perpendicularly or orthogonally (depending on what you have set for in the Control Box) to define the new opening edge.
This allows you to position a Door or Window with respect to an element within a room, or in another room, as shown below.