Java Question: Converting Hex String to byte Array when you don't know how many bytes are in the array?

I am interested in converting a Hex String to a byte Array, and having a method that will always work even if the size of the byte array needed isn't known.

There's a great answer if I want/need it to be only 4 bytes long -- but I want to understand better how to put that into a loop.

thanks,
mtitania