It this due to Captivate or to PPT? Sorry to ask such a question, but PPT has a weird way of dealing with 'size'. How do you define the size of a project in PPT? It is still in print units (cm or in) and it is very tough to find the exact size in pixels. How do they convert the cm in px, for which screen (in ppi)? All screen-focussed applications will define the size in pixels. It is strange for a presentation application that is meant to be viewed on screens, to use print units. I wonder how they define the font size as well? Captivate has to translate everything in the logical screen units which are pixels, not cm/in. With these reflections in mind, what is the 'actual size'? There is no absolute size on screens at all.
Each ppt-slide is converted to a 'movie' slide in Captivate, which makes editing very difficult. I never recommend the import PPT-slides workflow for that (and other) reasons.
Sorry, seems to be off topic. I have already asked quite a lot of times what the idea is behind defining a dia size in cm or in, which has no sense in my (logical?) engineer's mind.