A 5lb'er maybe a little too small and the 20 could be too big. The issue isn't the size of the extinguisher, it's knowing how to use it and not panicking during it's time for use.
I'd suggest you stop by an local extinguisher supply, talk to a sales rep and inquire about buying some refurbed units to save yourself some money. You might also be able to locate an older water extinguisher and use that depending onthe circumstances.
I suggest that you place them near the exit. If the fire is too big, continue exiting the building and call 911. If you believe you can handle the situation, grab your extinguisher and return to the fire. When in doubt, get out and stay out.