First of all, VIA doesn't operate HSR. In fact, I am unaware of any HSR lines that use diesel fuel, likely because of the significant increase in fuel required. They not only save weight by not having to carry the fuel, but they also don't occur the expense of buying all that fuel (electricity is much cheaper, especially when you consider the improved efficacy of an electric locomotive).
But, to answer the question, using some back of the napkin math, the energy density (energy per unit volume) of hydrogen varies depending how you store it (liquified or the pressure if compressed), but assuming compressed at 681 atm, 69 MPa, 25 °C hydrogen has an energy density of
5.323 MJ/L (HHV), which is less than 1/7 that of diesel (
38.6 MJ/L). VIA's SCV-42 locomotives have a 8,320 L fuel tank, so for the same amount of energy, the tank would need to be 60,000 L which is 60 m³. A Fuel cell is about twice as efficient as a diesel generator, so in reality you would only need 30 m³ of Hydrogen. Assuming a tank that has a 1m inside diameter (0.5m radius), the tank would need to be about 38 m long.
Venture coaches are 25.9 m long, so that means the fuel tank would need to occupy about 1.5 coaches to have the same range. For HSR, one would need to almost quadruple that, so probably 5 or 6 coaches.