Hi Marc
Automator might help, depending on how your images vary in size. For example, say your base image size is 3000 wide x 2000 high...... ( and vice-versa in Portrait mode ) You could set automator to filter all photos from a certain album ( or the whole library ) based on the height being greater than 2000 pixels.
Of course, this all falls down if you've cropped so that the height in pixels is less than 2000 ( for example ), but it might at least leave you with less to do manually?
if you've a wide variation in image sizes, one can get a bit cleverer in the same vein, and start with a width = less than 500 height = greater than 500 ( again example only, you may not need to go that small ) then run it again with increasing sizes ( excluding the sizes you already did...) but it gets a bit fiddly I guess & still requires some manual assessment of sizes beforehand.
Automator can create a new album based on the filters you set.
I have to say, having tested it out, that although it works, it's err quite ( well, very ) slow, & iPhoto carries on sorting long after automator says the workflow has finished; but if your images are mainly one size, just varying in orientation.... it's worth a go.
