[racket] numbers/bytes in r6rs

Hello

I'm struggling with chapter 2 of the R6RS libraries (relation between Scheme data objects and bytevectors). Does anyone know a clean&simple way for
- determining the number of bytes needed to push an arbitrary-sized number in a bytevector;  (log nbr 256) is not good enough :-)
- determining whether a floating point number is a double or a single.

I guess that I must be lacking knowledge about some of the primitives.

Thx

Wolf