Inevitable answer: "it depends".
A human uses about a hundred watts of power at rest, and maybe double that for extended periods if they're fairly fit. A world-class cyclist might manage as much as a kilowatt (possibly aided by pharmaceuticals).
The power-to-weight ratio of current fuel cells is quite variable, but by the time you've got actual androids to power I'm sure that the higher-end figures will be commonplace. A kilowatt fuel cell could, therefore, be only a kilo, and will probably fit inside a replica human torso, no problem. Even more pessimistic estimates aren't that bad, with a 10kg cell providing enough oomph.
Fuel cell vehicles are about 50-60% efficient. This figure probably won't rise that much in the future, so lets assume the worst for simplicity. Hydrogen has an energy density of 140MJ/kg, giving you a maximum of 70MJ of useful energy per kilo of fuel, or 19.5kWh. That's enough to run an idle human for over a week, or a world-class athlete running at full power for a bit under a day.
Modern high-pressure hydrogen tanks are filled at 700 bar. At this pressure, and at room temperature (handwave, handwave) hydrogen has a density of about 38kg/m3. That 1kg of hydrogen, then, could fit in a cylinder of 10cm radius and 83cm height. That's quite big! You'll have difficulty fitting that in a person-sized hull and still have space for all the other useful androidy things you'll be wanting.
Take home message: hydrogen fuel cells good, hydrogen fuel bad. I'm not sure what alternatives would be best in your situation, but obviously depend very much on what you want your android to do and for how long. Other types of fuel cell do exist with much more energy-dense fuel, but current efficiencies are low. Naturally, as the author you may handwave your future tech as you see fit. A methanol fuel cell with a 50% efficiency (current real world figures are more like 10%!) would be a good start... the fuel has 20 times the density of 700bar hydrogen and is a lot easier to work with. Even with a usable energy density of only 11MJ/kg, a 6.4kg fuel tank isn't a big load to haul around and fits in 1/3rd the space.