This can be done in Automate but you probably need an external tool to reduce the file size.
I don't think there is any possibility to affect the file size using a built in action. Unless you know of specific pages that can be removed then you can use the Remove action. You could use a command line tool like Ghostscript to reduce the file size. Then loop a second run and use PDF action Split to split the file.
Available PDF actions:
PDF - Add attachment(s)
PDF - Add header/footer
PDF - Clear signature(s)
PDF - Create
PDF - Create session
PDF - Decrypt
PDF - Delete attachment(s)
PDF - Disable field(s)
PDF - Encrypt
PDF - End session
PDF - Extract
PDF - Get attachment(s)
PDF - Get field value
PDF - Get field(s)
PDF - Get PDF information
PDF - Get signature(s)
PDF - Insert
PDF - List attachment(s)
PDF - Remove
PDF - Replace
PDF - Search
PDF - Set field(s)
PDF - Sign
PDF - Split
PDF - Stamp