TuneText uses an unusual way for saving results of readers' customization on local computer, - the Microsoft's userData Behaviours.
The userData is a special persistence framework for enabling users of browsers InternetExplorer 5.5 and up to save data on the client machine.
In the most cases the saving occurs "silently", behind the scene.

According to Microsoft, the maximum amount of UserData store depends on security zone as follows:

Security Zone	Document Limit (Kb)	Domain Limit (Kb)
Local Machine	128	1024
Intranet	512	10240
Trusted Sites	128	1024
Internet	128	1024
Restricted	64	640

Thus, allowed size of your changes is at least 64K for each TuneText pack.
Please note, here we are talking about size of your customization data. And each pack can consist of almost unlimited number of HTML files.