I don't know the real answer to this question (I would love that Tiki allowed bigger pages too).
In the meantime, you might be interested in using the INCLUDE plugin on a central page (as "master document"), and work on other smaller wiki pages (one per section of your long text, as "subdocuments" of the master document).
Since you will inlcude all subdocuments in the main page, you'll see the page as long as you need (on html)....
Is it clear enough? Does it make sense to you? (it works for me, at least)
Search for more info about PluginInclude , if needed (and "include" it in you tiki installation, if needed ;-).