How can I decrease parcel size while a posting is active?

[Anonymous]

Now that the postal service has starteed using new size standards to calculate postage, I need to put more accurate size on a parcel that is listed.  It would save the buyer a few bucks probably.  When I go in to Revise your Item, it won't let me revise the size and there is still 5 days left on the listing, but there are bids on it.