How wide? You can trace the max texture size your GPU supports by adding:
trace(FlxG.bitmap.maxTextureSize);
This property will only be available on non-Flash targets (like Neko).
As a general rule, Flash supports much smaller texture sizes than what your GPU on Windows/Neko supports (More if it's Stage3d, which is unlikely in your case), so I doubt your issue is really about how wide your texture is. However, on Windows, HaxeFlixel uses a different rendering system (
http://haxeflixel.com/blog/01-HaxeFlixel-rendering/) than what Flash uses (blitting) so maybe the problem is actually there.
There are many ways around many things, but they rely on the specific problem you are faced with: How wide are they? What are they for?
And I have no idea about the Windows tag.