There are two things that you will probably need to do remedy this issue:

For printing associated with your PC’s software applications, you’ll need to do the following:

1.  From your PC's Control Panel, double click on the 'Printers and Faxes' group. 

2.  Right click the Dell A940 printer, and select 'properties'. 

3.  Click on ‘Print Preferences’.

4.  From the 'Print Preferences' menu, change the ‘Paper Setup’ found on the left-hand side to ‘Letter (8 ½ by 11)’.  The current default is probably set to A4.

5.  Click on ‘Ok’.

This will change the printers default setting to ‘Letter’ size all future jobs printed from software applications.

You also need to adjust the paper size settings on the A940 itself for standalone purposes (faxing, copying, etc).  To perform this, do the following:

1.  From the front panel of the A940, press the ‘Options’ button two times.

2.  Use the left and right arrows to find ‘letter’ (the default will probably currently be set to A4).

3.  Press the ‘select’ button.

The default paper size will now be ‘Letter’ for all future standalone jobs.   Hope this helps!