It sounds like eBay may have automatically opted you into their new global shipping program which requires dimensions on items that previously you wouldn't need to indicate them for. I've read that eBay is automatically opting people in to that program without notifying them, so you may need to check your eBay settings to see if thats the case.
Here is a most recent article about eBay doing that on ecommercebytes:
http://www.ecommercebytes.com/cab/abn/y13/m08/i05/s01