max size of ost file if memory serves correctly is 2gb, you can actually convert ost file to pst file in outlook 2007, it require some effort from your side and will work only if your file is not damaged:
1. open outlook
2. go to "file"
3. "import and export"
4. choose "export to a file"
5. select "personal folder file .pst"
6. tick "ost mailbox" to select all folders and data within
7. select where yo9u want to save your new .pst file (desktop or root directory are best, easy to find)
8. click on "finish" and wait...
usually this is best method to cahnge but it have some drawback, contacts are sometime unfortunately skipped by this method, i bypass this problem by e-mail them to myself and then i have it as an e-mail.
another problem is that if your ost file exceed 2gb or is damaged, you need to use third party software to repair or truncate it, there is no easy way to verify it through any of the ms programs and there is no native utility within outlook or office to help you with... there is several line commands to recovery and divide ost file, but that exceed the scope of this short info
as i was writing this from memory, please forgive for any errors made
more info you will find in this m/s article:
i never used this program but it appears to be free:
if you download it - do not install any junk which most probably will come with it, let me know if it is worth to play with, if you use it.
if you find this info helpful - do not forget to vote for it...