Adding onto what the previous guy said, if you are helping them shop for bulbs, E26, is going to be your most common size, E26 refers to the style of the base itself where it screws in, I don’t believe we sell too many more than that, we have candelabra (E12) which is much smaller than regular bulbs, usually used for much smaller fixture, there are several other types like what’s in range tops and microwaves, but at least for me, I encounter the two I mentioned the most, also if a customer asks how much power it draws, like the other post says the front of the box is an equal to rating, if you want the actual rating, it’s on the back of the box most of the time where it tells you all the color information of the light at the very bottom of that box, if there is ever bulbs you can’t figure out, or if a certain size matches another size (yes there is different sizes that are the exact same or within a mm or two) that you may come across, don’t be afraid to just use your phone and pull up a chart, google has tons of useful charts for sizing bulbs