Some numbers. Ordinary petrol has an energy density of 46,4MJ/kg. Total allowed fuel now is 110kg. Total stored energy 5104MJ. At 50% TE this means in one race maximum 2552MJ goes to the wheels. Equals 708kWh.

Suppose an F1 race is 90 minutes this means you will need a 472kW fuel cell. If you can regen more during braking the size obviously decreases, that would be nice from a technical point of view however I don't think you will get the stopping power they have now.

Suppose they manage to make the fuel cell 75% efficient incl balance of plant. Would mean there is 944kWh potential energy needed. Hydrogen lower heating value is 33,3kWh/kg so 28,5kg of liquid hydrogen is needed.

Some notes: radiator size isn't only depending on the thermal efficiency but also the delta T between operating temp and ambient. The larger delta T, the smaller the radiator size. If you have liquid hydrogen onboard it might make sense to use it to cool the FC.

Battery size will probably need to be bigger than current since there is less power from the FC available vs ICE.