Size
The amount of data that you can back up is limited to the amount of space that you purchased. You can back up files of any type or size with only a few exceptions.
Mozy Common Blacklisted Files on Windows
Mozy Sync folder
Mozy 2x Protect folder
Some of the Mozy program files
\$Recycle.Bin
\Pagefile.sys
\System Volume Information
\hiberfil.sys
For a full list, see C:\Windows\mozy[pro].blk (Text File).
Microsoft Common Blacklisted Files
Outlook OST Files
For a full list, see this Microsoft Article and HKEY_LOCAL_MACHINE\System\CurrentControlSet\Control\BackupRestore\
Installed Programs
Mozy can backup exe files, however Mozy can not reinstall them. It is recommended that you backup the source data for your programs, and the installer. There is no reason to backup the installed program itself, and may cause problem.
Encryption
Officially the only supported encryption is EFS when enabled in the settings.
Otherwise you may try to backup encrypted data, but it may not backup or restore correctly or successfully, and there may be unforeseeable behaviors.