In addition to the directional arrow on the air filter, you will also see numbers denoting the filters width, length, and thickness. This is the air filter size your HVAC unit takes and should be the size you always order. Using the correct-sized filter is also important so your filter can do its job properly and not allow dirt and debris to bypass the filter. It should fit in the slot snugly, with no major gaps on the size and you should never have to force it into the housing.