One nice feature of AutoCAD® Architecture is that you can assign standard sizes to Doors and Windows. This allows the user to select a standard size from a list of the sizes entered into the Style, saving time and minimizing the potential for error.
Given the wide range of things that grips on AEC objects can do, I highly recommend turning on grip tips, if you do not already have them on. Check the toggle in the Grips area on the Selection tab of the Options dialog or set the GRIPTIPS system variable to 1.
When you first make the grip hot, you will be restricted to stretching the grip to a standard Width, as defined in the Style. ACA will display a series of lines, indicating the places to which you will be permitted to stretch the Window or Door. You can stretch to any Width that is part of a standard size, even if it is not paired with the current Height as a standard size. To show this graphically, "long" gray lines are used to indicate Widths that are paired with the current Height in a standard size, and "short" red lines are used to indicate Widths that are part of at least one standard size, but which are not paired with the current height in one. A grip tip will appear to remind you of this, if grip tips are active.
If you press the CTRL key once after making the grip hot (but before left clicking to select a stretch point), you will be able to stretch the grip without any restriction.
* - Unfortunately, if the current Window Width and Height are not a standard size, the tip for the Width grip will read "Custom Width", even if that Width does appear in the standard size list, paired with a different Height. The tip really should read "Standard Size" or "Custom Size", to be less confusing.
NOTE: Images in this post were made using ACA 2010. Other versions may have slightly different graphics or text.