For some reason, Version 1.4 almost doubled in size to 47KB. I'm unsure what's new or what's changed but 47KB is huge. Sure, TinyMCE is 110KB but not much can be done about it. At the moment I'm using the full 1.4 package but I'm considering just backporting the 1.4 feature and sticking it onto 1.3.1.
It may even be a good idea to generate a cut down version of Prototype for our uses since we don't use 80% of it.
Prototype.js does make stuff a lot nicer but at 47KB per visitor, it's not worth it.