An incorrectly sized air filter can cause a lot of problems. When it's too small, dust, mold, and other contaminants can enter the vents and be released into the air, leading to air quality issues. Filters only work if air has to pass through them, and if there is a gap of 1, a disproportionate amount of air will flow through the hole, making the situation worse. Yes, you can cut off an air filter if you know what you're doing and if the filters you use aren't weakened too much from the cut.
For example, HEPA filters offer exceptional filter quality and are commonly found in hospitals or similar areas that require the cleanest air. If you live in an older home with a non-standard size air return frame, you can use foam weatherstrip tape to slightly increase the dimensions of your preferred filter or order a custom-sized air filter. In these cases, you have more flexibility in selecting the size of the filter since you can choose one that is longer than the size of the opening. This is especially true if you have a HEPA filter installed in your HVAC unit, as these filters truly outperform even dedicated air purifiers when it comes to removing particles from the air.
If it's choosing a smaller filter or nothing at all, at least do something like taping or taping a piece of cardboard filler to the filter to get the right size. These filters can block larger particles such as dust, pollen, and carpet fibers, but not many others. If you have to jam the air filter to install it, you should try a different brand of oven filter or a custom-sized air filter with a slightly smaller actual size. While the filter is an essential component integrated into the HVAC unit itself, an air cleaner is a separate unit that can be configured together with the rest of the system.
Traditional flat panel air filters are the least expensive but also least effective. While some air filters fit perfectly without any modification, this may not always be the case, especially if you have ducts that deviate from standard designs. If you're particularly concerned about indoor air quality but don't want to break the bank with a more technologically advanced purifier, an extended media filter should work well without interfering with normal HVAC operation. Keep in mind that affordability comes at an additional price as they don't filter as well as more expensive options.
This also gives you a pseudo “handle” that sticks out of the opening and allows you to grab it for easy removal when it's time to change the filter.