Having recently experienced a fire in a friends Exige [ I was driving at the time] caused by a broken oil line fitting [ more details soon] I would recommend that you go for the largest size Extinguisher you can fit and with a minimum of 4 nozzles in the engine bay .
The system I am looking at is a 4L [10lb] 4 Nozzle Electric for the Engine Bay FIA 4.0L Electric System : Racetech New Zealand
And a 2.25L [5lb] 2 nozzle for the cockpit ...this might be a manual setup.
I need to do more research for the best position to mount the nozzles.
I am not concerned about the increase in weight !!
The fire was fuelled by 8 litres of oil from a broken oil fitting [ dry sump] that splashed oil onto very hot headers.
The car did not have a plumbed in Extinguisher system. Luckily it happened on the main straight near to the pits so plenty of fire fighting equipment available . It took 3 large hand held extinguishers to put out the fire.
If the car had a plumbed in system I could have activated this earlier and the fire may have been controlled earlier ... as against having to wait until I stopped and the marshals reached the car.
If this had been a track day and I had stopped on the other side of the track I doubt the fire could have been extinguished.
The other thing I learnt was to ensure the you have a well placed, large sized Battery Isolator. The engine was shut down by the ECU when it triggered an alarm for low pressure , but the Battery isolator switch was a small button ( see attached pic] that I had trouble turning off. Had to have a couple of trys to get it turned off .
The other thing is to practise your emergency escape procedure ... especially if new to the car !!
Good news is that the car is repairable ... even the clam ... and we hope to be back and running in a few months.