The small mouse dropping chilies are what really make Thai food fiery hot, buts it's often the much larger and more visible spur chilies that get the blame. Spur chilies are usually around five centimeters (two inches) long, and can be found in red, green and even orange colors.
Red and green spur chilies.
The spur chili's purpose is often more decorative than functional. It is frequently used as a garnish to add colour to a dish, although the chili's own heat should not be under estimated. Many northern recipes call for roasted chilies, and the spur chili is quite good for this. Use only the ripe red kind, since they are easier to peel once roasted.
The Thai spur chili is directly descended from the Tabasco chili used to make the famous American sauce. So, if you are unable to find the Thai chili, try to find Tabascos.