I've got Azuras Star with a grand (ogre) soul captured. I want to enchant a couple of shoes with shield for my mage. I am only able to add 10% of shield to those shoes.

What determines the effect of the enchantment, is it my level? I know it's also the size of the soul, but ain't a grand soul pretty alright?

Grand Soul is the best and the only factor in this. 10% is maximum when it comes to shield enchantments

If you want, read more about it http://www.uesp.net/wiki/Oblivion:Enchanting

Yeah 10 is the highest with a soul gem, but I think the transcendent sigil stones offer 20% but you have to be a somewhat high level to get them

If you're going with a sigil stone I'd use an elemental one like fire shield, since this adds both shield and resistance to fire.

This is a good idea. I have a spell which gives me 20% shield of every element. In total I get 60% shield to damage plus 20% shield against fire/frost/shock :hehe:

edit: not 60% since I don′t have 100% spell efficency

Not to mention that those Sigil Stones give you 25%, whereas the plain Shield ones at the same level are only 20%. That's true at all levels, the elemental shield is more shield plus resistance.

Wow I never realized that, I need to go update some armor

I just got a sigil stone last night with my new character at lvl 5 or 6. 13% frost shield, gives you shield and the destruction resistance. best way to go.