Dear Carroll and Cedric,
I have had exactly the same problem with slideshows on my two sites, now with HostExcellence (see for example http://briangrassom.com/Painting.html
and other photo pages on that site). This only began recently, so I'm not sure if it is coincidental with upgrading to Lion 10.7.4, or moving from the MobileMe server to HostExcellence.
But I have checked in the 'sites' folder opening the 'index.html' file on my Mac from which the websites are uploaded via Cyberduck to HostExcellence, and the problem is there as well: so, it looks like it may be something to do with iWeb (I have version 3.0.4) and/or Lion? - particularly if Carroll has the same problem and has not yet moved to HE.
The slides will display full screen (in which they are set in iWeb's 'Inspector') immediately upon opening the slideshow, but invariably they will display much smaller (too small) if the show is re-opened in the same session, or if I move to another page's slideshow. It does not matter if the slideshow is on the 'photo' page, or in an embedded 'album' page. In any one session the slide size in all pages will be erratic and unpredictable. The same thing happens if I view the website on our iPad.
I wonder if more people are experiencing this issue just now, and if it therefore has something to do with upgrading the OS system to cope with the current emergence of iCloud and demise of MobileMe: or if it is something technical, limited to a few users and their particular website designs?
Thanks for reading this rather long missive.