> What if you don't know ahead of time how big that monitor is that you are displaying stuff on?

Use a reasonable upper estimate?

> ad-hoc re-implementation of virtual memory?

If you rely on actual virtual memory instead of specially designed file format, saving large files will become prohibitively slow. On each save you have to stream the entire document from page file to actual memory, serialize the document, produce the entire file, then replace. And then when resuming editing after the save, you probably have to load the visible portion back from disk.