How big could a hydrogen dragon get while still looking and acting like a dragon?
From comments & answers to this question 'not very', apparently a 1,000 ib race horse would need something like 377 m^3 of hydrogen to fly.
I'd like a creature at least big enough and strong enough to carry a fully-grown human.
One small problem with that .. the more lift your applying to the animal with (what are for all practical purposes) helium filled balloons, then the less agile it's going to be .. howsoever if being able to carry a person about is all you're really concerned about?
Then I offer you my answer to to this other question.
Specifically Quetzalcoatlus northropi.
Ok, so there is this (see below).
'But when it flew, the Earth spun a little faster (so gravity was effectively (ever so slightly) less), the atmosphere was also thicker (making flying easier), & the atmosphere had a higher oxygen content (making muscles more efficient) ... so Quetzalcoatlus might not be able to fly today.'
But those are relatively small quibbles & can probably be hand waved away with just a small amount of hydrogen unlikely to impact it's appearance too much, fill it's hollow bones with it, give it a few gas bladders elsewhere filled with the stuff & wave your hands while you tell the crowd you've nothing up your sleeve & the jobs a goodun :)
In Summary :
So really big arse balloons aside (which definitely impacts on it "looking and acting like a dragon").
I think Quetzalcoatlus (with or without just a bit of hydrogen in play to assist the handwavium) is likely the best model there is for the largest a flying animal can plausibly be in Earth like conditions.
Some Other Links :
Quetzalcoatlus, see the section on flight.
I did look for a YouTube of Paul MacCready's 1984 model northropi ornithopter but wasn't able to find a good one, besides his was only a half size model, this one is full size but fixed wing & unfortunately it's more a 'making of video' than anything else with only few shots showing the thing really flying (the tether was also rather short so those shots there are don't look all that impressive).
Lifting Properties of Hydrogen :
A cubic foot of hydrogen lifts around 30.8 grams (68 lbs per 1,000 cubic feet).
Considering a hypothetical 7 foot long 2 foot wide 4 foot high helium filled bladder along the spine of northropi (dimensions that hopefully won't detract overly much from the dragon look & aerodynamics) as an initial back of the envelope calculation gives us 56 cubic feet .. so around 3.8 pounds of lift.
So even filling it's hollow bones with hydrogen you probably can't plausible get away with much more than 4 or 5 pounds of lift from hydrogen without seriously impacting the appearance of your dragon.
For Reference :
An adult human is 137 pounds, the Kori bustard 25 pounds & Argentavis was maybe 156.5 pounds.
Quetzalcoatlus we really have no idea but estimates have ranged anywhere from 150 to 550 pounds.
Person Carrying Potential :
The wingspan of Quetzalcoatlus is comparable to an ordinary hang gliders (often also around 10 m).
Tandem hang gliding is a thing.
So the idea a dragon the size of Quetzalcoatlus could swoop down in a shallow glide & snatch up someone from the ground is very plausible.
But that's with it already in the air & up to speed .. trying to get airborne from a standing start on flat terrain with the extra weight of a human body might not be possible & will be a serious struggle for it to achieve if it is (having a steep hill or cliff to jump off is another matter of course).