Setting sprite custom size in the instance properties can fail if you set an animation != 0 in the properties too, with a different final scale 
In the editor (different animations from the same sprite object, resized):
In the browser (as you can see, the first animation has the right scale):
I’m sorry, surely it would take me a lot of time, and as you can see in the closed pull requests on GitHub… I have not mastered it 