Question: how big are led lights?
Answer: LEDs come in all sorts of sizes as well. Here is a photo showing a 3mm, 5mm and 10mm LED. the "millimeter" size refers to the diameter of the LED. For example, if you need to drill a hole in a box for your 5mm blinky LED, the hole size should be 5mm, and you'd need a 5mm drill bit to make it.