If there was a software-based emulator, that could do it, but all the NES emulators on the GBA use the GBA's 2D hardware to recreate the NES's native drawing functions, which is the reason games run at full speed. This hardware emulation means that while you can scale things vertically, you cannot scale horizontally, and that's just due to the quirks of the GBA's 2D hardware which you would need to know more about coding to truly understand, so just take my word on that for now.
Check out the very bottom paragraph of http://pocketnes.org/res.html and the page it links to and you'll see that even if you could do it, you really wouldn't want to.
The version of NES DS that uses software emulation might be able to scale proportionately, but I do not know this for a fact.