I guess what you want is the number of characters in a decimal representation of the binary floating-point double-precision number. Unfortunately, such things are dubious, because it's all inexact.
Perhaps, though, all you want is the number of characters in Java's default way of outputting a particular double. In that case, you can just convert the double to a Double, do toString() on it and get the length() of the String.
Betty Rubble? Well, I would go with Betty... but I'd be thinking of Wilma.