How do I enter the fuel tank size?
 

Without it, it's impossible for Fuelly to give an accurate MPG figure but I can't find where to enter my car's fuel tank size.

It's going to different every fuel up, you simply enter how many litres you put in the tank each time you fuel up. Has to be brimmed though, otherwise fuelly can't do the maths.

Ooh, so it doesn't need to know the fuel tank capacity to work out the MPG?

It needs to know much fuel you've used obviously, but that could be different every fuel up, if you fill up half way, it's obviously going to be half the amount from filling it from empty isn't it. And not filling to the top is marked as a partial fuel up, it won't be able to work out your MPG from that because it doesn't know how much fuel you've used.

I always fill my tank up to the top (auto cut off of the pump) but of course it's never empty when I start to refuel as that would be stupid (as in: risk).

Of my 60l tank, my recent fills have been: 58.5, 58.2, 55.1, 55.3, 55.9 (all showed the red light fuel indicator on the dashboard).

I presume, based on the responses I've received above, knowing the maximum capacity of the tank is not required for an accurate MPG reading?

I have no idea how much my tank holds. I fill up until the first auto cut off of the pump. If I have done 250 miles since the last time I topped up, and it takes 5 gallons, then it returns 250/50=50 mpg. That is how Fuelly works. Fuelly allows Trip miles or Odometer miles (you must use one or the other - you cannot swap back and forward). I use Odometer miles as that allows me to reset my trip meter for a particular journey, if I wish.
I usually run fuel down until the fuel warning light comes on, but not every time. The more top ups you record the more accurate the overall mpg becomes.

It doesn't matter how much the tank can hold, only how much you used since your last fill. If you stop at the automatic cut-off every time you fill the tank and put that amount in it will be pretty accurate, as close as you can really get unless you used some really sophisticated flow meters etc. that nobody would ever use.