Very cute story! Keep writing them!
Crossovers are so hard to do - they either don't fit, or they just seem too forced, and the characters don't really work together from the two universes. So this was impressive that you were able to do a crossover, and make it work.
As far as the length, I think it's just right. Anything much longer would have been unfaithful to the Scooby Doo fandom. In a way, this is a Scooby Doo fanfic with a Hellboy insertion (not the other way around), so being true to Scooby Doo length and levity was really the way to go. Good job!
Just a little nitpick. Chapter 2 needs a little editing. "And packed" should maybe be "And they packed." And this “Will you two knock it off? I ain’t got any mutant mutt treats to feed you to convince you to get in" seems awkward...
But other than that, I really enjoyed this ficlet. I'd say it should be followed up by a Nancy Drew x Hellboy, but honestly, I like that it isn't, because that ending was just so funny.

ok great storie i cant stop laughing. please please please .finish it.

Hilarity indeed! I started laughing at chapter 1 and I hadn't stopped yet! Please keep writing.

Hellboy and Scooby Doo...lol XD

Ahahaha. Brilliance, that's what this is. xDD
Update?

i'd be knocking my head against a pillar too, if i was stuck with them D:

isn't it creepy how they've managed to look so young over all these years? They should be at least fifty by now...maybe it's something HB should look into. ;o

luv the story, definitely gonna read moar

awesome chappy! i can't wait to read more!

Very cute, but please, longer chapters.

Man, I'm gone for a little while and you come up with a story invovling Hellboy and the Scooby Doo gang...awesome...I am already very very interested.

I think the story is good, but I think you need to make it longer so there will be more funny stories

Hilarious, but I agree that this is too short. More soon, please... Beth Palladino

Great update, but please, longer chapters. More soon please.

muahahaha! this is amazing! u hvae me laughing so hard! i'm definitely loving this! update soon!

that was amazing! scooby! but seriously very funny!