Let's take your common NES/SNES resolution of 256x224. Regular 8x in each direction would give you 2048x1792. Keeping vertical resolution would give me a resolution of 2389.333333333333x1792 aspect ratio corrected. Since YUV 4:2:0 in AviSynth requires the resolution be divisible by 4, the closest resolution would be 2388x1792. Going by what I said, with Luma being resized twice, you would first resize the RGB to 1/4 of 2388x1792, which is 1194x896:
Language: avisynth
AVISource("ExampleClip.avi")
chroma = PointResize(1194, 896)
Then resize Luma a second time:
Language: avisynth
luma = chroma.PointResize(2388, 1792)
Finally combine the Luma and Chroma:
Language: avisynth
luma = luma.ConvertToYV24(matrix="Rec709", chromaresample="point")
chroma = chroma.ConvertToYV24(matrix="Rec709", chromaresample="point")
last = MergeChroma(luma, chroma)
I should note that I've not tested this, but this should work in theory.