I have a friend who owns a fire extinguisher business, and we sometimes spend some of our free time testing expired, but still perfectly serviceable fire extinguishers.
Here are some of the things I have found:
It's only too big, if you can't run with it. Those little 2 pounders are not going to get you too far, if you are not right at the point of use, when the fire starts.
A 10 pound unit is as small as I would go, unless you physically can't fit anything that big there. I like having 20 pound units in my shop, and house.
Get in as close as you can before pulling the trigger. Most common household extinguishers do not have much discharge time. Anything you shoot in the air while approaching the scene, isn't going to scare the fire that much.
The most overlooked piece of advice on using a fire extinguisher, is to take a deep breath of as fresh air as possible, and hold it, before discharging your fire extinguisher.
If you just ran to a fire, or are just breathing hard from all the excitement, and in the rush to put the fire out, you neglect to take that deep breath, you will end up inhaling a bunch of powder, from the cloud you are standing in. All you will be able to do then, is cough, and gasp for air rendering you mostly unable to fight the fire.