Hi MarleneM (& Others),
Actually, eBay appears to have recently changed how they handle the entry of package details (such as dimensions) since I last replied to this thread such that the Sell your Item form now allows this information to be entered regardless of which shipping service or package size is selected - and even when only offering flat rate shipping.
Perhaps the inconsistent behavior between the Sell your Item form and Revise your Item form I observed before was a product of the changes that eBay has been making to this part of the listing creation process.
Our Product Management team is currently looking into this change and I expect we will be able to adjust our lister page to correspond with these latest changes regarding the entry of package details as part of a future update to our site.
-Mike