How to determine how many bytes an integer needs?

Use this:

int n = 0;
while (x != 0) {
    x >>= 8;
    n ++;
}

This assumes that x contains your (positive) value.

Note that zero will be declared encodable as no byte at all. Also, most variable-size encodings need some length field or terminator to know where encoding stops in a file or stream (usually, when you encode an integer and mind about size, then there is more than one integer in your encoded object).