Size of my fist or size of dust,
Color of night, rainbow or white
If I check health,
I'm unhealthy myself.
I can be hot.
Ur eyes will water,
At my incredible bite.
What am I?
Answers
pepper
Size of my fist or size of dust
A whole bell/capsicum pepper is roughly fist-sized. Ground-up black pepper seeds are dust-like.
Color of night, rainbow or white
The pepper seeds we use ground for seasoning are usually black or white. Chili peppers and sweet peppers are commonly red, orange, yellow or green. (I have yet to see a blue one, so maybe "rainbow" is stretching it a bit.)
If I check health,
I'm unhealthy myself
I confess I'm not sure what this is about. After writing all the rest of this answer I happened to look at the riddle sandbox and saw a mention of "the Dr. bit" at which all became clear: "Dr Pepper" is the name of a soft drink; doctors check your health but soft drinks are usually not at all good for you
hot
There are "sweet" peppers with no capsaicin in them, hot chili peppers with lots of capsaicin, and black/white pepper seeds that come from an entirely different plant and are hot for different reasons.
Or eyes will water
At my incredible bite
Capsaicin from Capsicum sp., and piperine in Piper nigrum.