The smallest I've seen is the fanz flexr, which is just over the size of a single AA battery with a fan each end.For fans you can go to expense with a paintball specific fan, or get small 20mm computer fans and wire one or two to a battery and switch
Fans about 10 years ago made a lot of noise for not a lot of air flow. I'm not familiar with the current paintball specific goggle fans, so can't say if they have improved or not
The only issue I have is there is no protection for it and at £50 ago, if it takes a paintball hit its a gone.
But I might take a look into adding some small fans to the top vents, I've got to do something as its ruining my games as I can't see anything after the first 2 games as I'm too hot by then