If you are using Outlook 2003, you can find the size of your PST file by following these steps:
1. In Outlook go to menu File --> Data Management
2. Then select the "Personal Folders" file that starts with the Outlook.pst file name by clicking on the entry.
3. Next, click on the "open folder" icon above the link.
4. You'll see the contents of the folder, including your PST file. Right click on the file and select Properties and you should then be able to see the size of your Outlook file.