Ugh. Hena, I don't care if it's standard paper or not. (insert Joe vs. the Volcano quote) I'm not arguing that with you. Epic Raiders and Raider 2.0 are Standard US Letter. Epic: Armageddon and Epic: Swordwind were some size smaller than that. A4 is bigger than all three supplements. If a smaller size was good enough for GW, it is good enough for me.
You can make up your supplements in any size you want. It doesn't matter to me. But don't expect me to print it if its A4 size or larger. It's a pain in the rear to pack properly and I don't see you jumping on a plane from Finland to run to the Post Office for me.

Seriously though. I put very few stipulations* on printing (ask E&C) and I think making these US Letter or smaller is an itty bitty request that anyone needing my services can handle.
* In case anyone is wondering what I need from a supplement before it goes to print, they are based on a combination of needs from GW and me. Here they are:
-No mixed IP (i.e. pictures of GW models next to DRM models)
-Make it available on the net for free (pdf downloads)
-Include GW's no challenge to IP disclaimer
-Boldly marked as unofficial
-Adhere broadly to GW's IP guidelines in terms of the fiction (no Space Marines acting like tree hugging Eldar, etc.)
-Include a small symbol on the backcover for my non-profit purposes
-Try to print close in size to the existing supplements