Official mGBA Ports

[SuperrSonic]

Is there a way to turn off the filter?
Is there a way to make the screen smaller?

While we wait for such features, you can try Extrems' version which has those two settings. Plus instead of bilinear it uses oversampling so it looks just as good as if it were point filtered. There's a funky color thing going on that mimics the system being used during daylight, not a big fan, and there's no way to turn that off.