I just noticed an apparently-new warning in the "Mobile crawl errors" section of Google Webmaster Tools. It reports that two pages on my main (non-mobile) Web site are too big for common mobile devices, with the error message "Document size too large".
These two pages are quite a bit bigger than all the others, so I cannot tell where Google's cut-off is for determining that a page is "too large" for mobiles, but it is somewhere between 21kB (Size O.K.) and 30.3kB (Size too large).
Unlike other errors and warnings I've seen, there's no icon next to the warning to click to see the documentation for this error, so I guess they haven't documented it yet.
The good news is that I limit my mobile-device pages to 17.5kB, well under their threshold... :)
Thanks for narrowing it down as close as you already have. I also cannot find any official documentation about limits to document sizes for the mobile crawl.
We are probably on the cusp of a mobile explosion, thanks in good part to the success of the iPhone. I expect many more of us will be looking a lot harder at mobile browsing and mobile search in the very near future.