determining sample size

Dear colleagues,
I am a biologist and i need some help. i am planning to conduct a study where i am trying to identify a marker for a disease. the marker (independent variable) is a protein that i can qauntify in saliva and it is continuous variable. the outcome is disease or no disease. my hypothesis is hat the protein can indicate the presence of disease. I want to test whether my protein is significantly associated with the disease so that i can use it as diagnostic non-invasive marker. I have at my hand 2000 cases and 7500 controls.
What is the minimum sample size do i need to get a significant association between my protein and the disease.
The protein has never been used before the purpose i want to use it for.
thanks in advance

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