There are 4(5) different types of shields in the game. Each type generally dictates how it's blocking behavior is.
Size 1's are the smallest, such as a Buckler shield. They have the highest block frequency, but they also provide the lowest damage reduction.
Size 2's are the next size up from size 1's. They are round shields like the Numinous or Master shields. They block less often than size 1's, but they reduce more damage.
Size 3's are the next size up, and are the main shield type. They block less often than size 1's or 2's, but they also reduce damage taken by a significant amount. An example of this size is the Kite shield, or the Koenig shield.
Size 4's are the largest size, and they block infrequently. They are not highly reliable. However they offer the largest damage reduction of all the other sizes. An example of this size is the Tatami shield, or the Tower shield.
The final size is the relic shield Aegis, which has the damage reduction of a Tower shield and blocks as frequently as the size 1 Buckler shields. Some incorrectly label it as the 5th size, but since Aegis is the only shield at this size rating I don't think it really deserves its own size rating...unless you really want to say it as such. Lol
Hope that helped!
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If you need a bard to get "good exp" in a merit party, you're the weakest link.