I've never really captured my own souls, and it now astounds me. When I played on the PC version I just spawned a grand soul gem using the console whenever I wanted to enchant something, or used sigil stones.
Now I play on the xbox version, so obviously I can't cheat and spawn things.

I picked soul trap as the spell for my mages staff. When I encounter something, I now cast that before whacking whatever it is attacking me. I also have Azura's star. The trouble is, I always seem to encounter a message that says "that soul is too large" or something like that, and it never captures it. The only soul I've been able to get is when I killed a wolf, and that was only a lesser soul. (Azura's star should be a grand soul gem right?)

Can anyone help? Is it even possible to soul-trap humans? My character likes to enchant things to aid her in her adventuring and sigil stones have unpredictable effects.

Grand Soul Gems and Azura's Star cannot trap NPC souls. An NPC is basically anything that can talk, like Humans, Elves, Dremora Lords, etc. NPC's souls are fundamentally different.

For that you need a Black Soul Gem. You find out about Black Soul Gems in the Mages Guild quest line.

But Azura's Star or an empty Grand Soul Gem is large enough to trap any creature's soul.

Black Soul Gems is what you want. For sure. If you want an endless supply of them, use the 'duplicate item' trick with scrolls. You just need to make 1 black soul gem. As long as the BSG is empty, the scroll trick will work.

Good Catching...

Azura's Star is a reusable Grand Soul Gem. This means that once you trap something in it, use it to enchant orr recharge so that it's empty for the next use. Remember that souls will go into whatever empty gem is available at the level of the soul or higher (i.e., a Grand Soul Gem will trap any level of soul [not humans, though], a Common Soul Gem will trap Common souls or below, etc).

You probably still have a soul in the Azura's Star, so you need to empty it to use it again.

Actually it says right on the screen what the effects are...

I think she means the fact that they are RANDOM, but your post is very informative, none the least.