"..adjust each frame's size individually.."
D'you mean size as in dimensions (..640x480 pixels..) or as in duration (..time it stays onscreen)..?
I don't know of a way - but that doesn't mean there isn't one! - to automatically adjust the dimensions of a set of separate photos (..if that's what they originally were..) apart from, say, using an
Automator script or AppleScript to automate a particular set of actions ..assuming that the program which you want to automate (..Graphic Converter? ..PhotoShop? ..Preview?..) accepts scripts.
If you want to automate, or convert a whole set of,
durations of transitions (..i.e; make all Dissolves 3 seconds long..) then the only way I can think of doing that is as I described above.
Unfortunately, I can't remember the characteristics of iMovie 4 ..and it might take me a while to dig out mine (..I keep only 2 and 6 regularly to hand..) but I know that I was rather disappointed by it, so that's why I upgraded and largely forgot it, sorry!
QuickTime Pro, which I mentioned (..and it costs $29.99, and just upgrades your existing QuickTime Player by a simple click on the QuickTime tab at the top of this page..) has a facility to export a set of pictures as a movie sequence, with the individual frames set to whatever duration you want, as far as I remember. But as I said; Karl knows much more about that than I do ..give a shout and he may come and reply..
However, I don't think that will automatically dissolve between the frames ..though maybe an automatic dissolve can be integrated by using one of those scripting methods I mentioned above.
As you can tell, my forté is not animation (..though I used to do it long ago with 8mm film ..oh; and more recently with a little Olympus Pen 35mm half-frame camera..
..because a 35mm half-frame is identical to a 35mm
movie full frame, so that's a quick and
cheap way to make full-size cinema film animations!) ..But that's the limit of my knowledge..