Sample Size with small population

Hi eveyone,
I'm intending to investigate a factors affecting a small population and I need help with my planning as I'm not well versed in statistics.
I'm looking at a group of students totalling 41000 nationwide, the subset which I intend to investigate makes up 1% of the total. After looking at some ways of analysing it looks like I should use a binary logistic regression, with the 2 Y axis variables being part of subset or not part of subset, and the variables being the different factors affecting them- is that appropriate?
Then I need to work out how many people need to respond to this survey to make it significant, I have looked at sample size calculators but I can't make sense of them, can anyone help me?


Ambassador to the humans
So you want to know what might cause a certain person in subgroup to either apply or not apply to your university?
I would only be looking at the people at the university, so would be comparing barriers which people in subgroup have overcome vs students not in subgroup. I'm trying to see if people in subgroup have overcome more barriers to access university (therefore explaining the small population). I hope that makes sense!