time complexity – Modification to the Algorithm merge sort

I was trying to solve this problem but I’m struggling in the time complexity
Can u help me please:
Consider the following modification to the Algorithm mergesort. We apply
the algorithm on the input array A(1..n) and continue the recursive calls
until the size of a substance becomes relatively small, say m or less. At
this point, we switch to Algorithm insertion sort and apply it on the
small instance. So, the first test of the modified algorithm will look like
the following:
if high − low + 1 ≤ m then insertion sort(A(low..high)).
What is the largest value of m in terms of n such that the running time
of the modified algorithm will still be Θ(n log n)? You may assume for
simplicity that n is a power of 2