Which air filter do I get?

Jimmy said that it was no wonder that there was no cooling going on with our a/c system… He wanted to work on cleaning out and blowing all the dust and dirt out of the cooling system system and so he wanted me to go to the hardware store to pick up a new air filter for the a/c system

I must admit that my fiance does way more around the house than I do. Of course, I do lots of housework and laundry and cooking, but my fiance Jimmy does all of the home improvements and all of the repairs around here. for instance, Last week our a/c stopped working for some odd reason, and there was just lukewarm air coming out of all the air vents and the house just wasn’t cooling down. We had the a/c turned all the way down on the thermostat, but nothing was happening. So Jimmy went down to the furnace room in the basement to see if he could figure out what the problem was with the cooling system… I heard him messing around with the central a/c in the basement, but there still wasn’t any cold air coming out. Jimmy called up the stairs to get me to come down from the dining room. I went downstairs to the furnace room and Jimmy was holding up a disgusting looking air filter. It was black with dust and dirt and it looked like it was completely clogged up. Jimmy said that it was no wonder that there was no cooling going on with our a/c system… He wanted to work on cleaning out and blowing all the dust and dirt out of the cooling system system and so he wanted me to go to the hardware store to pick up a new air filter for the a/c system. I left to go to the store, but when I got to the heating and a/c aisle, I saw that there were hundreds of filters that would fit our a/c system. I had no idea which air filter to get.

Propane boiler