Question: If n is very big, and k is very small, can I say that O(kn) is linear complexity?

What if k is closed to n/2, but not more than n/2? Do I consider it still as linear complexity? Or quadratic complexity O(n^2)?

Is there a limit to how big k is, to consider O(kn) as quadratic complexity?

Answer: 
If `k` is a constant, then any O(kn) function is O(n), i.e. linear

If `k` is a function of `n` and is O(n), then any O(kn) function is O(n^2). n/2 is O(n). Furthermore, `(n^2)/2` is *not* O(n), and so if `k` is close to `n/2` then `kn` is not O(n).

If `k` is not O(n), then `kn` is not O(n^2).
Assuming that k and n are independent variables, saying O(kn) is linear is a proper statement.