So I know, or at least think that if the level of oxygen doubled it wouldn't have a huge effect but it would increase one distance running and if it was high enough that we could be physically bigger. But what if it did not effect us in size and we are the same as we are here on earth, At what level would oxygen cause harm to a human being for example When does it becomes toxic.
Googling "Oxygen Toxicity" returns several fairly informative results regarding how too much oxygen can be dangerous. In particular, from this PDF, the main mechanisms of death due to oxygen toxicity are:
Central nervous system toxicity – the 'Bert effect'
Occurs at somewhere around 3 atmospheres partial pressure
Pulmonary toxicity – the ‘Smith effect’
Occurs around 0.5 atmospheres partial pressure over prolonged periods
Regular air has an oxygen partial pressure of around 0.2 atmospheres, so doubling that partial pressure should still be survivable for most people, but much further would mean heart failure and death. Additionally, there are other nonfatal effects of oxygen toxicity:
Infants who are exposed to atmospheres of over around 40% oxygen in the first few days after birth are at a very high risk of developing a condition called Retrolental fibroplasia, in which an opaque membrane forms behind the lens of the eye, leading to blindness.
So, in an imaginary world with double the oxygen, everyone would be blind from birth.