Recessed lights are categorized by their size. The most common residential sizes are 3-inch, 4-inch, 5-inch and 6-inch.This leads to the common question; "What is this size is actually referring to?"
The answer is it's the size of the inside diameter of the housing (aka "can") with the trim or LED module removed.
A common mistake is to measure the outside of the trim which will give you a larger (inaccurate) reading.
How to Measure What Size Recessed Lights you Have
Remove the light's trim or LED module
Measure across the opening of the inside of the housing at the widest point.
A Few Notes
Housings sizes can vary a little between manufacturers, but they should measure close to a whole number - either 3", 4", 5", or 6".
Many LED Retrofit modules come in 5/6-inch size which means they have an adjustment that allows them to fit either 5-inch or 6-inch housings.