1.5 kilo BCF
Now that we have lost the wonder that was halon and a decent affordable clean agent hasn't emerged it's either powder or foam.
Powder has the plus point of the best mass for mass extinguishing power, important with small units, but has the usual issues of vision obscuration (hazard to traffic as well as the operator and really bad if you discharge it whilst in a vehicle, hence it's prohibition with PSV's and minibuses) no cooling and no flashback protection so you have to get it all out in one go, not always easy with vehicles and spilt fuel.
Foam allows partial extinction, cooling and gives flashback protection, plus doesn't have the vision & associated problems. But if it's a flowing fuel fire you are in trouble (unless you have a lot of extinguishers*) and requires operator skill as it's lower mass for mass power means a low capacity extinguisher is soon gone and any wastage vastly reduces what you can achieve.
The 1 litre is popular with at least one police force, several local authorities for taxis and some minibuses, PCVs and similar and despite not being an EN3 compliant size, the models on sale generally say "Manufactured in accordance with EN3, PED, CE" or similar and have the CE mark meaning that non specialists in extinguishers will accept them even for PSV purposes.
My main gripe with the 1 litre is it is too small for any meaningful fire fighting capability.
Size wise I'd go for nothing less than 2 kilo/2 litre size, although I've effectively used a 950g aerosol BC powder in the past.
In a perfect world you would have 2 extinguishers, powder for knock-down and foam for cooling and post knock-down security.
I have a 2 kilo ABC Powder in the footwell and a 6lb Monnex and 3 litre foam in the back.
* The difficulties using foam alone on a flowing fuel fire:
http://www.youtube.com/watch?v=PAgFK5M2PRM
