I think that I have mentioned this before a few years ago. Anyway, sizing pixels are an unofficial work-a-round to ensure that multiple objects of varying shapes maintain a fixed postion and size in relation to each other. The problem with these added 'pixels' is that they are never truly invisible and can become very unsightly. I suggest using a box in Xtreme etc. instead. The box would frame all your vector shapes. It would be visually easier to use than sizing pixels. It would be ignored visually by X3D as it is made up of only lines (X3D only recognises solid shapes and not lines).

Yours Anas