Easy way to configure the page size in Google Docs™ to any of the A sizes:
4A0, 2A0, A0, A1, A2, A3, A4, A5, A6, A7, A8, A9, A10 and also important envelope sizes: DIN lang (DL), C4 C5, C6, C6/5

Please don't ask me to add other page sizes, as the purpose of this extension is to add support for the page sizes commonly used in Europe.

You can fork this extension and create your own, if you like. It is Open Source.

Google Docs does not support custom page sizes, with this addon you can set any of the A* page sizes in portrait or landscape orientation.

Size		Width x Height (mm)	Width x Height (in)
4A0		1682 x 2378 mm		66.2 x 93.6 in
2A0		1189 x 1682 mm		46.8 x 66.2 in
A0		841 x 1189 mm		33.1 x 46.8 in
A1		594 x 841 mm		23.4 x 33.1 in
A2		420 x 594 mm		16.5 x 23.4 in
A3		297 x 420 mm		11.7 x 16.5 in
A4		210 x 297 mm		8.3 x 11.7 in
A5		148 x 210 mm		5.8 x 8.3 in
A6		105 x 148 mm		4.1 x 5.8 in
A7		74 x 105 mm			2.9 x 4.1 in
A8		52 x 74 mm			2.0 x 2.9 in
A9		37 x 52 mm			1.5 x 2.0 in
A10		26 x 37 mm			1.0 x 1.5 in
DL		220 x 110 mm
C4           229 x 324 mm
C5          162 x 229 mm
C6          114 x 162 mm
C6/5      114 x 229 mm

This addon only sets the page size and page orientation. Use File → Page setup to set all other page settings. As that dialog does not support custom page sizes it will reset your page size. Simply use the add-on menu again to set your custom page size.

Note. This add-on is meant as a short cut to quickly set any of the DIN A page sizes (A1, A2 ...). If you want to set an arbitrary page size please use https://chrome.google.com/webstore/detail/page-sizer/acgkneeneageffjinlglednnehpelffb