You should strive to write your code without special cases.  Having more code paths makes your code harder to analyze.  Actually, your general case already works to cover everything.

What is the point of `count`?  Isn't it always `i - 1`?

While `for(int i = 1; i <= size; i++)` is not wrong, it would be more idiomatic in Java to write `for (int i = 0; i < size; i++)` unless you had a good reason to count starting from 1.

Personally, I prefer a ternary conditional to better express the fact that one way or another, something will get printed.  That's a matter of taste, though.

    public static void xo(int size) {
        for (int row = 0; row < size; row++) {
            for (int col = 0; col < size; col++) {
                System.out.print((row == col) || (row + col == size - 1) ? "x"
                                                                         : "o");
            }
            System.out.println();
        }
    }

If you care about performance, then you would be better off building the entire result first, then printing the entire string in one `System.out.print()` call.