You need to reduce the overall .pst file size by using any of the below methods:
1-You can delete the unwanted email and other items from .pst file. You must empty the Deleted Items folder
2-If the issue persists, you can compact the file that can help to reduce its size
3-Alternatively, you can create a new .pst file and archive the email items to it
4-You can also use a powerful PST File Splitter that could split your .pst file into several .pst files. These software perform Split PST operation by using advanced and safe techniques.
Stellar Phoenix PST File Splitter is a powerful application designed to provide safe .pst file split. It is a safe PST File Splitter that can split the Outlook .pst file into different small .pst files, each usable with Outlook. You can split the .pst file on the basis of sender's email ID or date. It supports Outlook 2007, 2003, 2002 and 2000.