Was looking at some early medieval manuscritps and realized there was a bit of a size discrepancy sometimes, which got me thinking. How much did these actually evolve over the early Middle Ages ?
Obviously there were probably some different umbo designs (still gotta find a good terminology for that btw) and paint jobs, but i'm moreso curious about things like the size, curvature, etc...

One needs to be cautious about early medieval manuscript illustrations of shields. For example, how much are we seeing actual shields and how much stylised images based on Late Roman originals? To what degree do bowl shaped shields illustrated capture reality?

A rather older monograph on Anglo-Saxon shields which may still be of use is

https://www.academia.edu/477692/Early_Anglo_Saxon_shields_Archaeologia_Monograph_110_London_Society_of_Antiquaries_of_London_1992

A lot of work experimentally on shield construction has been done by Scandinavian archaeologists, which has also looked at the archaeology, especially about bog finds. These publications tend to be easy to find online.

This article on Slavic shields also provides a slightly different perspective and some interesting archaeological finds

https://sagy.vikingove.cz/en/defining-slavic-shields-of-9th-11th-century/