To the OP's question, I think it's a matter of how big of a space it is and how it is configured (i.e. how many obstacles are in the way from any given point between you and an extinguisher). You may be better off having several smaller 5lb. units at multiple locations rather than one large 20lb. The smaller units are easier to hang in a crowded shop or garage than those 20lb. beasts. They tend to be cheaper also even if you have to buy several. But in any event these should be re-chargeable, gauged units; Amerex or Badger matters little although Amerex are more expensive on average but have a better reputation. You should also check with you local fire extinguisher service company to see which they service as it might save yourself a headache down the road when you have to re-charge them.
I have 5 units in the hangar (shared auto shop and storage) not because of the size (2,000 s.f.) but because there are 8 cars, parts storage, workbenches, etc. that block your path to the single big 20lb. CO2 unit. The other 4 are 5lb. rechargeable Badger Halotron/Halon units all at logical places throughout the space (all with clear signs and/or reflective stickers set high above the roof lines of the cars so you can see them all). 2 of them are right up front at the large folding door in case they are needed outside on the taxi way as well. This doesn't account for the 1.5lb Amerex Halon units in the car trunks that are less accessible but there just in case. The idea was to always have one within 5-10 steps to wherever you are in the hangar. This may be overkill, but fire is a big concern since there is no fire sprinkler system inside the hangars and we are all joined together with common walls across 4 hangars.
In a my home garage/woodshop (+/-350 s.f.) I have one 5lb. BC standard dry chemical at the side entry door. But it might be on the Kidde list so it may be time to replace it with something more substantial. I have a 10lb. Amerex ABC dry chemical rechargeable inside the house (Laundry Room) and a small 2lb. Kidde(?) ABC under the Kitchen sink (it too may be on the list).
The only kicker to this is getting them all recharged and/or re-certified every couple of years. Not to mention making sure the CO2 unit's valve and tank are sound. I have to do this to ALL the rechargeable units right now which means a few trips to the service shop over the next month or two.
However, this is a small price to pay for safety as well as peace of mind.
