An overall measurement is to make the shield 3 feet or a little more in height (1 meter or so), and about 2 feet wide. However, remember that Celtic inspired shields came in all shapes and sizes, so you have some freedom to choose. There is certainly no one rule about their size!
If you want a clear cut historical example, look here:
http://www.myarmoury.com/nateb_othr_nate_shield3.html
Another description mentioned that this particular shield was:
