int highNum = 0;
int m;
int list[4] = {10, 4, 7, 8};
for (m = 0 ; m < size ; m++);
{
if (list[m] > highNum)
highNum = list[m];
There is a ;
right after the closing parentheses of your for loop:
for (m = 0 ; m < size ; m++);
The statements inside the block (inside the curly braces) gets executed only after the loop do nothing for size number of times and that too only once.
You have also missed a pair of { ... }
for the if statement as well.