From ArcGIS Mobile team, "At ArcGIS Mobile 9.3 the Map cache has been renamed the Mobile Service Cache. Each layer in the Mobile Service Cache can store up to 4GB. We have also implemented a Mobile Base Map as a data source to the Map. The Mobile Base Map is in effect a single layer in the map. It can be built by using an ArcGIS Desktop GP tool that takes as it's input one or many layers from a MXD and compresses the data into the new format. This Mobile Base Map layer has a limit of 4GB.
Note: the 4GB limit is a fixed limit and it is not recommended that this exact amount of data is stored. It is best to test each scenario especially for Windows Mobile where RAM limitations can restrict the amount of data that can be read effectively. This could be as little as 250MB. When deploying onto a Windows 32-based device it is important to leave some room especially in the Mobile Service Cache where you might want to store field edits. For example, don't try and load exactly 4GB into the cache. It would be best to load just 3.5GB to leave room for any field edits."
There is an existing enhancement request related to your question - #NIM039193; Documentation is required for Mobile Service Cache and Mobile Base Map size limits. Please contact Support Services and log an incident so that we can add you to NIM039193. Thank you.