Are there any standards on the dimensions of PFD symbols?
Yes there are standards. There are industry standards which not regulatory standards, but they have been deemed acceptable by the regulatory agencies (e.g. FAA). So following them is usually accepted without issue.  

The FAA also provides guidance in the form of an Advisory Circular, AC 25-11B Electronic Flight Displays.

You have two separate things to address; the actual display hardware and the displayed content.  

The standard for the hardware is SAE Aerospace Standard AS8034B, Minimum Performance Standard for Airborne Multipurpose Electronic Displays.  

The rest is covered under a group of SAE Aerospace Recommended Practice (ARP) documents. The top level is ARP4102, Flight Deck Panels, Controls, and Displays. Under it are several Volumes and Appendices.  

ARP4102-7, Electronic Displays, covers the general requirements for what must be displayed.  

ARP4102-7 Appendix A, Electronic Display Symbology for EADI/PFD, covers the details for the PFD. Appendix B covers the EHSI/ND and Appendix C covers the Engine displays.