also, it must not be too heavy for ur family members to carry.
make sure everyone knows where it is and it is accessible, dont go put up inside cupboard that kids cant reach it.
take some lessons at civil def. centre eg. fire station to know how to operate & where to aim, not knowing where to aim may delay the escape.
note the expiry date.
Just to add on further, you should be aware of the class of extinguishers available. It defines the type of fires that it can be used to extinguish.
Class A:SOLIDS such as paper, wood, plastic etc
Class B: FLAMMABLE LIQUIDS such as paraffin, petrol, oil etc
Class C: FLAMMABLE GASES such as propane, butane, methane etc
Class D: METALS such as aluminium, magnesium, titanium etc
Class E: Fires involving ELECTRICAL APPARATUS
Class F: Cooking OIL & FAT etc.
Fire extinguishers are also Colour Coded for type of fires and medium used:
Water - Red
Foam - Cream
Dry Powder - Blue
Carbon Dioxide (CO2) - Black
Halon - Green (now 'illegal' except for a few exceptions such as the Police, Armed Services and Aircraft).
For home use, normally the Blue coloured extinguishers can be use for type A, B, C, E, F fires. It contains a dry powder. Do note that it usually lasts between 1-2 mins of continous discharge, depending on volume of cylinder. NOT much really, importantly is the technique applied.
The "sweeping" technique of spraying over the fire is used to smother the fire. This basically done by sweeping and lopping the powder over the fire. Don't shoot directly on the fire as this will splash and spread the fire, thus worsening the condition. A level head is also required.
C02 extinguishers (Black coloured cylinders) might leave no mess as Limsgp mentioned but that is not the best advice. Messing up your home might be the last thing in your mind when a fire breakout. Anyway C02 fires is ONLY effective in an enclosed room where the concentration of the C02 can stunt the fire by depriving it of O2. Usually ONLY used for type E fires - Electrical fires.
And yes fire needs Oxygen+Fuel+Ignition source to occur. Eliminate anyone of those and the fire will die.
Lucky for me, I work in the Petrochem industry and part of my job scope is as a trained Auxilliary Fire Fighter to fight refinery scale fires . I can apply at home if need be.
Importantly if the fire can't be control, evacuate - equiptment and money can be replaced, lives can't.