It depends on your playstyle mostly. Smaller shields are great for parry reliant builds, tower shields tend to tank more damage. So if you prefer to parry and get the parry bonus by following up with an attack, you want a small one. If you prefer to stand there and face tank the damage until an opening presents itself (or can't quite time parrys correctly), tower shield might be the better option for you. Overall it can also be situational. I barely use shields at all yet I got a serpent scale shield around the iron age because I had to cross a plains to reach the main swamp I was farming and didn't feel inclined to repeatedly die to deathsquitos for being in the wrong place too early.