Check your printer settings through CUPS:
If you print all of your data on A4, you can change the paper size at
Set Default Options.
If you only print some of your documents on A4, and some on 8.5x11, you will be better off creating a 2nd printer (with the same destination). Also, since one can specify the name the printer goes by, use a more mnemonic name.
On MY system (YMMV) I have 3 "printers" defined:
deskjet with 8.5x11 paper,
envelope with #10 envelopes, and
Labels8163 with type 8163 labels. All three point to the same physical printer. In normal operation,
deskjet is active and the other two are paused. I use
cupsenable to switch among them.