Read some links. It seems like using the computed style is still a recommended way. This discussion for example:

http://stackoverflow.com/questions/7404457/offsetheight-and-offsetwidth-calculating-incorrectly-on-first-onclick-event-not

has an experimental claim that "if you do not define a height and width for the element you wish to determine the height and width of, you will not get the correct height and width using offsetHeight and offsetWidth."

Also, the definition of offsetHeight/offsetWidth says that they exclude the margin of the element. I'm not sure if that's important.

Also, I'm setting the updated size via element.style.width/heigh, so it seems sort of more symmetrical to get the original size from the style as well. If one of you has more insight on this, let me know, but for now I'll leave this as is.