Is there a difference if I use a petty soul gem to enchant my daedric armor instead of a grand soul gem? Will the enchantment be stronger?

The bigger the soul gem, the better the enchantment.

Just use Grand gems, with Grand enchantment. Right one if it costs 500 gold. Or same for Black gems, at 750 gold.

One thing you should be aware of with soul gems. If you soul trap a petty soul and the only gem you have in your inventory that is free is a grand. It will fill the grand but the gem will be the power of a petty when you go to use it. So what I do is try to keep all types of soul gems in my inventory that are free to avoid this. Good to know.