[Will Kee]"Is there any way of customizing resolution when rendering to Sony XAVC on Vegas Pro 13."
No. XAVC is an industry standard like AVCHD, XDCAM HD, XDCAM EX, etc. They are guaranteed to follow certain specifications.
If you want to create your own frame sizes you'll need to render to something a little less standard which doesn't care what the frame size is. Try using MainConcept AVC using the custom frame size setting and change the width and height to 2048 x 853and it will render fine.