Filter SDK/Change frame size

From Avisynth

The following example code shows how to double the width of the destination frame:

public:
        ShowPixelValues(PClip _child, IScriptEnvironment* env) :
        GenericVideoFilter(_child)
        {  

// constructor code

        vi.width = vi.width*2; // width is doubled here
        vi.height = vi.height*2;  // height is also multiplied by 2

}

    PVideoFrame __stdcall GetFrame(int n, IScriptEnvironment* env);
};  
PVideoFrame __stdcall ShowPixelValues::GetFrame(int n, IScriptEnvironment* env) {
        PVideoFrame src = child->GetFrame(n, env);
        PVideoFrame dst = env->NewVideoFrame(vi); // new frame is double the size of src
        ...

A similar approach has to be used for changing colorspace, framerate etc. There is a bit more information at Working with images.