I purchased these for a condo furnace - they are too long but easily cut to fit the furnace. Worked well last year, and I replace annually so as soon as furnace is cleaned, we will insert a filter. Highly recommend.
I don't really know what to look for in a filter, but find that when I change them every 3 months, they are suitably filthy, so I assume they are doing their work! I learned about Filterbuy when I purchased my home and the previous owners left a few. When I ran out of those, I checked around, and Filterbuy had great ratings. The order arrived quickly, and now I'm set for another year!