Add one pixel of transparency around your image.
Also, it seems like pixelSize is not relatively consistent across an effect stack, one way around this is to set the opacity of the object to 99%, so the effects in the effect stack then have consistent pixelSize. Otherwise, you may need to use different H and V scaling for the blur.
Port of C2 effect from TheBen:
construct.net/en/forum/extending-construct-2/effects-31/effect-giant-blurs-adjustable-84139
Also use BigBlurH for a full blur.