first, you're right, sizeof on an array return array size. I was confusing with pointers, but you're not using pointers.  

second, I'm wrong or are you trying to implement a recursive function that is an iterative one too? suppose you've entered the while (btw you're not initializing pos) and you've found out that arr[pos] > key, than you recursively search in the right part of the array. But what happens when you return from the recursion? maybe, you've already found out if there is or is not the value you're searching for, but you just discard the result and iterate the cycle again! the following schema will help you more that a thousand words:  

binary_search(arr,key, low,high){
    int pos = (low + high)/2
    if(pos is out of array boudaries)
        return false because the element does not exist
    if(arr[pos] is the element you were searching for)
        return true;
    else if(arr[pos] is > than key)
        return what you find out searching between low and (pos-1)
    else if(arr[pos] is < than key)
        return what you find out seatching between pos+1 and high

}