My question is whats the difference in using various soul gems to enchat your item.....i mean in morrowind you needed grand sould gems to enchant a constant effect item but in skyrim you can use any soul gem to enchant non weapon items.....whats the difference?

I've noticed that the bigger the soul gem, the higher/longer the charge to the enchanted item.

That may be but what about the constant effect of items that are always on....whats the difference?

The Magnitude of a Clothing/Armor Item with a Fortify Affect is greatly Improved by the Quality of the Gem and Soul trapped inside of the Gem.

The Magnitude is also affected by Relevant Perks taken in the Enchanting Tree.

For constant effect enchants, like say muffle, you can use a small soul gem and it would net you the same effect. The size of the gem/soul has more impact on items with a magnitude and charge factor.