There are a lot of free printable stationery items on this site, but not many envelopes. Hence I’m making an effort to add more printable envelopes, starting with this 4.8 inch square envelope template. To use this, print the pdf file and fold the flaps toward the centre of the paper. Smear some glue on the side flaps and press the large bottom flap firmly down to stick.
Yes, 122 millimetres is a peculiarly size. The reason why is down to considerations of what best fits on a sheet of printed paper. To compensate for the worldwide uses (depending on where you live) of both A4 and US letter size paper, I compromise by downsizing my printable sheets to 8 x 11 inches. What this means in practical terms is there is ever so slightly less printable space, thus making it difficult to create a square envelope template that is any larger.