Microsoft have announced changes to pubCenter ad sizes and new targeted ad support.
For Windows 8.1 there will be a new SDK, but there is also a new version of the 8.0 SDK.
If your current Windows Store (8.0) apps use the following ad sizes you’ll need to replace them and resubmit your app:
In addition to removing these sizes, there’s a new size of 300x600, this means the new sizes you should be looking at are:
Steps to Take
Even if you you don’t use these sizes you may still want to update to the latest ad SDK.