help on wcscpy_s - size of buffer

It seems that you are just started programming C/C++, because your code shows some misunderstandings and will probably result in buffer overflows. But everybody was a beginner once and made such or similar mistakes.

A proper implememtation of your code would be:

size_t remove_space(wchar_t *equation)
{
    // length of input string, buffer size is at least len+1
    size_t len = wcslen(equation);
    // allocate buffer that always fits
    wchar_t *equation2 = new wchar_t[len+1];
    size_t eq_index = 0;
    for (size_t i = 0; i <= len; i++)
    {
        if (equation[i] != L' ' && equation[i] != L'\t')
            equation2[eq_index++] = equation[i];
    }
    wcscpy(equation, len+1, equation2);
    delete [] equation2;
    return --eq_index; // return length of string
}

However, removing chars from a string may be also done without using a help string. Another option is to use a string class providing a remove function (e.g. CString and Remove).