So I keep getting the error message "No Soul Gem Large Enough" when I kill humanoid enemies after using both the soul trap spell or with a weapon enchanted with soul trap. Thing is, I have empty petty, lesser, common, greater and grand soul gems in my inventory. Am I missing a certain type of soul gem needed?

Is there already a soul in your bigger gems?

If you're killing NPCs (as opposed to monsters), you need black soul gems.

I've completely ignored this aspect of the combat, but my best guess would be that Black Soul Gems are now needed for human souls.

Maybe?

You need a Black Soul Gem for black souls( Argonians, Elves, Khajiit, Humans), everything else can be used for white souls(animals, and stuff)

Trapping humanoid (human, elf, beastfolk) souls (aka "black" souls) is impossible with conventional soulgems. This wasn't even possible at all in TES II and III, (although in II vampires classified as "non-humanoid") but TES IV introduced "black soul gems," which are capable of holding these souls. They are technically considered a step above "grand," since they'll ALSO hold any souls a grand soul gem would. (but this would be a waste)

your not missing too much though, from what I understand, in Oblivion Black Soul Gems were about 10 percent more powerful than Grand.

Ahh, thanks for the help guys! I really only started playing the series with IV and I didn't recall having to use Black Soul Gems to capture humanoid souls. So I guess the next logical question is, where can I find some Black Soul Gems in Skyrim? I'm level 23 and haven't found a single one yet.

Go find Azura's Star... It's a fun quest! It is essentially a Grand Soul Gem...if you are trying to find black soul gems...go into caves. I found one in a random cave. I'm at a loss of the name at the moment though. *thinks hard*

They were pretty rare in Oblivion, so I'd bet on something similar here.

(Also, at least in OB, black soul gems were considered evil/corrupt/etc, since you're trapping the soul of a "person", not some monster, animal, or demon. The argument over whether that's screwed up morality has filled up a good number of threads. )

the mage's college has a dude who sells you some 'underhand' wares if you go to his room... his name escapes me

he has a black soul gem in his inventory, about 1300 gold or so iirc..

Rare to already find them as a black soul gem...If you had grand soul gems you can just go to Dark Fissure Cave and make some! But sadly...we are not in Cyrodiil anymore.

They don't seem nearly as rare here, actually. I've found several lying around in random caves.

Check the Jarl's wizard in each city, they usually sell them.