You can typically use Grab on creatures of up to your own Size category unless your specific Grab ability text says otherwise. The Feral Gnasher archetype for Goblin Barbarians gains the ability to use Grab on increasingly larger creatures as the character gains levels. Nowhere in the Pathfinder rules though can I find any information regarding a size limit for using Grapple. I'm guessing that either:
A) There is no size limit. You can try to Grapple whatever you'd like, but the CMD of big, strong creatures might make it difficult
B) Although the rules don't say it, the expectation is that you can only Grapple creatures of up to one Size category larger than yourself.
If situation B is (or "should be") the case I wonder how the Feral Gnasher's ability to Grab larger creatures would interact with that. I'd guess that maybe it should allow the FG to use Grapple on larger creatures as well. For instance, it would be kind of silly if a 9th level FG could use Grab on Large creatures but couldn't actually grapple them. A few older threads I dredged up seemed to favor interpretation A, but I never saw an official ruling or developer comment. I'd expect to experience some table variation either way as DMs might decide that even if a goblin rolls a nat 20 to grapple he can't stop Cthulhu from moving.
There is no size limit to grapple. That i am Aware of.
Capt. Darling, you are correct: there is no size limit. There is also no penalties for grappling larger characters outside the normal CMB/CMD adjustments for your size.
I still can't find any size limitations for Grapple in the RAW. I'll inform the DM that it looks like the Feral Gnasher will be able to at least attempt a Grapple on any monster she cares to throw at us.
With a fully buffed grappling CMB of +25 at 9th level I think his chances of success should be pretty good against all but the biggest and baddest of foes. Having a Witch with Evil Eye in the party might help too. I wonder if she has Ray of Enfeeblement and what the biggest creature we can conspire to grapple might be.
Grab only works on the sizes specified but you Can use a normal grabble on purple Worms and bigger:)
And remember to get strength surge so the first grabble becomes easy.
Can you choose to add the bonus from Strength Surge after you know the result of the grapple check, or do you need to apply it up front?
Up front.