Welcome to MobileRead.
In answer to your exact question, there is no way to make DOC files page break at a specific point. All new page or section break commands in the DOC file are ignored (read unsupported) by the Reader. Likewise, TXT files do not support the new page command.
The only way to achieve your goal is through the use of other formats such as PDF or LRF files. As Jon pointed out, the Sony screen is 90 mm by 120 mm so making a correct sized PDF is a simple matter. On the other hand, the speed and flexibility of an LRF file can be an asset and use of the free tool BookDesigner should be considered. There is a great storehouse of knowledge about BD in the MobileRead community so assistance is just a question away.
Good luck and keep us informed on your progress.