I've always thought that a shield's hp was dependent of character height. Because it has to englobe them fully at full health. However I think there is an attribute that ends up defining the hp. For example, olimar's shield won't break with a CS, but megaman's did and it was much larger than a full olimar shield.
As for breaking shields, I've noticed that samus is one of the better (and only really) characters that can break shields semi consistently. Although shield regeneration is ridiculous in this game, samus has both hard hitting attacks and they can easily be stringed together. As mentioned above, charge shot, super missile, up tilt, dair, bair and bombs do amazing shield damage. While other attacks like ftilt, uair and regular missiles are good for slightly poking the shield safely and to continue to mix things up for attack. It's pretty cool when they actually survive the charge shot with just a fraction of the shield but the hit stun won't let them let go of the shield so it ends up breaking anyway.
Also, screw attack does ridiculous shield damage, perhaps even more so than an charge shot, but it'll only fully connect when an opponent is on the top platform on battlefield. It'll most likely hit them first than breaking the shield, but get an up air in there and then a screw attack and it'll get very close to breaking while also launching the opponent most likely.
By the way, when at 80% you're obviously wanna go for the charged f smash. But low percents I'll go for an up tilt combo and at a little higher percents go for the dair to get something going. Against larger character, is fsmash better or would up smash have more ko power?
which is bad because that's the MU I know the most because of that. 
