Help- what should I do about my dog?

We got a new dog today that we'd applied to adopt. I absolutely love him, as does the entire family. However, I think I made a wrong decision. While he is so sweet and a fantastic cuddlier and adorable, he really just isn't big enough and playful enough for our lifestyle. You have to realize that this absolutely devastates me- he's the best dog anyone could ask for, and two others families had applied to adopt for him- the entire shelter staff said they'd get him if they had space, even a few crazy cat ladies! But I already spent the money on an appropriate crate and absolutely everything he needed, a lot of it specified for him specifically and I doubt I will get full returns- I spent a LOT of money, and if I got a different dog I'd have to spend a lot more. In fact, he's sitting outside the door right now while I go to the bathroom- is that a fantastic dog or what? I just want something bigger that we could use for what we want to do. I promise you I will cry if I give him up, but he isn't what we wanted. Also, I'm afraid we'd get a bad rap with shelters if we returned him. PLEASE help me, I'm desperate.