iFrame Size Override in ExhibitBuilder

I have no trouble using an iFrame in an Exhibit page after following all the lovely discussions in the forum.
However, when I paste in the HTML and click update, I cannot seem to alter the iframe dimensions. When I update page, I can see the revised dimensions in the HTML and even in the TinyMCE editor, the iFrame size is graphically what I hope to achieve.
When I click update the size always reverts to: width="320" height="240". The iFrame is fully operative, just postage stamp sized.
Has anyone else experiences this. What perchance may I me missing? Thanks for any assistance.
Am using Omeka 2.3.1 and ExhibitBuilder 3.2.1.

Do you have iframe.width and iframe.height specified in your "Allowed HTML Attributes" setting under Settings -> Security?

Thanks John. I should have thought that one through. Bang on and sorted.
Much appreciated.