I am not completely clear on what the issue is with your markup. Are you attempting to change the markup for existing items where the size is not what you desire?
In any case, you must not mix using XElement (LINQ to XML) with the strongly typed OM for the Open-Xml-Sdk.
The best way to research the markup that you need to change is by creating two copies of a document, changing one slightly, and then using the Open-Xml-Sdk Productivity Tool to compare the two, highlight the differences.
In the following screen-cast series, you can watch screen-cast #13:
Introduction to Open XML
Best, Eric