Envelopes are usually available with three different types of seals. In order to avoid any confusion, here is a short explanation of the most used terms:
What is the difference between envelopes on the one hand and mailers on the other? An envelope on which the flap is on the short side is called a mailer; if the straight or pointed flap is on the long side it is called an envelope.
The most common sizes of mailers are as follows:
DIN C5 162 x 229 mm
DIN B5 176 x 250 mm
DIN C4 229 x 324 mm
DIN B4 250 x 353 mm
DIN E4 280 x 400 mm
Size and position of the DIN window:
DIN C5, B5:
The size of the window is 45 x 90 mm and its position when the mailer is horizontal is on the left (flap on the right) with a 60 mm margin from the bottom and 20 mm margin from the left side.
The size of the window in 55 x 90 mm and its position when the mailer is upright (flap at the top) is on the left side with a 212 mm margin from the bottom and a 20 mm margin from the left.
Because the post office is constantly changing the maximum size and limitation parameters, we refer you here to the hopefully always current Deutsche Post webpage: www.deutschepost.de. There you can also consult the German post office postage calculator (only in German although some info in English can be found on the Internet).
Similar webpages for other countries can be easily found in the Internet