Vord length distribution

[Koen G]

You should think of a diphthong as one vowel, which is why it makes one syllable.
The difference between a regular vowel and a diphthong is that when pronouncing a regular vowel, your mouth remains the same: aaaaaaaa. 
But when pronouncing a diphthong, you need to move your mouth a little bit. For example, when you pronounce the "ou" in "sound", the diphthong travels from the back to the front of your mouth.

The "oo" in "cooperate" is not a diphthong, these are two syllables. You say [ koh-op-uh-reyt ].
The first "o" in "cooperate" is a diphthong, it should be pronounced with a w-like sound at the end. Even though this is pronounced as a diphthong, it is written as one letter.
The "ay" in "say" is a diphthong; you don't pronounce the "a" and the "y" separately, they glide as one sound.
The "ea" in "bleak" is a digraph, two letters represent one sound.