In most cases a 6 inch recess can uses a 5 inch diameter bulb. There are a couple option that include BR40 and PAR38 light bulbs. You need to make sure you know if the recess can you are using has a trim bezel that is designed to use a long neck bulb (BR40) or a shot neck bulb (PAR38). You also may have a recess can that allows you move the socket to set the depth according to what light bulb you want to use.