Ah, sorry it wasn't much help. :(
Regarding texture size, relaxguy mentioned in this page that it's device dependent. The haxeflixel documentation also talked about it in the mobile targets page.
I've never tried anything beyond 1024x1024 for texture size, so I can't tell you if going beyond 8000 is okay.
You can't do it directly with the haxeflixel library but it can be done with openfl. Check out this guide it might be helpful: http://www.openfl.org/archive/community/programming-haxe/integrating-admob-openfl/