IIRC, no Spark code ever "reads back" the size of display object. The scrollRect's width/height is computed from other properties. I think Flex in general rarely reads back the display object's size if at all since that value can contain stuff that isn't desired. I do recall a one-frame wait on MX TextArea to update its scrollbars, but I don't think there were too many others if any.
Also, in case it helps, in general, MX components used masks and Spark components used scrollRects.