I have yet to see specs or guidelines on running html banners in standard placements. The closest I get is this link to Doubleclick's html5 rich media guidelines.
So far, the animated HTML5 banners I've seen are all over 100k. We got a batch sent to us that were built using PNG sequences taken from the flash versions, stitched together using Jquery, Edge, and who knows what else, adding up to over 2mb each for what had originally been 40k flash files. This is not what Steve Jobs had in mind when he disallowed Flash from iOS devices!