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. 5mm are the most common size you'll see, and they can be extremely bright!
 
SPECIFICATIONS
Superior weather resistance.
5mm Round Standard Directivity.
UV Resistant Eproxy.
Forward Current (IF): 30mA.
Forward Voltage (VF): 1.8V to 2.4V.
Reverse Voltage: 5V.
Operating Temperature: -30℃ to +85℃
Storage Temperature: -40℃ to +100℃