Smaller shield texturepack allowed?

I'm not sure, but probably no because it gives some advantage.

RESOURCE PACKS RULES:

If the resource pack does not grant any advantage to the player and/or doesn't change the game considerably, it may be accepted if the moderator verifying the run finds it acceptable, but because it's a subjective matter, it is greatly encouraged to not use any.

Pretty sure that would count as an advantage.