Munich - Paper sizes
The basic printer paper and folder paper size in Germany is A4.
Most "Schulblocks", school exercise books, and lecture pads are also A4. The paper is different though. Instead of having ruled horizontal lines printed across the paper, most exercise books have both horizontal and vertical lines forming a grid.
Smaller exercise books are available in A5. A "Vokabelheft", vocab book, has a line ruled down the centre of each page, presumably to write German words on one side and "foreign" words on the other.
Vokabelheft are also available in A6, as is an "Aufgabenheft", homework diary.
Art paper is available in multiple colours and thicknesses. The commonly available sizes range from 50x65cm, 50x70cm to 70x100cm.
Envelopes come in a range of sizes. Business envelopes are "DIN Lang", and measure 110x220mm. If you wish to send a bunch of A4 paper without folding it, buy either a DIN C4 envelope or a DIN B4 (which is a bit bigger). For general letter writing, envelopes come in DIN C6 114x162mm or DIN B6 125x176mm.