Labels are preset at 4" x 6". Use this as a guideline and any printer should work.
If you need to print directly onto envelopes (using first class postage), a standard laser or ink jet printer should work fine. Simply insert an envelope into the printer as indicated in the printers manual (usually centered in the paper tray), and the postage will print directly onto the envelope. The "Paper Size" should be set at "Env10" and the "Scale" at "Custom" "100".
You can reprint labels over an over to try different settings. However, do not mail more than one of the labels, as this may be a Federal Offense.