The following answer is for those who are using Chrome. Other browsers have similar features.
For an email in ActiveDEMAND, click the preview button to show the email in a the browser. If you are have an email in your inbox, click the 'view in browser' link on the email. Or if you have a landing page, click the live page link or the preview button to get the page to render in a browser.
Once you see the image in the browser, right click on the image and select the 'inspect' option
Google will show you the 'in page' rendered size of the image
To see the 'actual' size of the image, choose the 'open image in new tab'
Then on the image in the new tab, do the same steps as above.