I believe that the RSA MD5 checksum is always 128 bits (someone correct me if I'm wrong). That means that it is 16 bytes long. However, when storing an 'MD5 checksum', you'll generally store it in some encoding, generally hex. If it is hex encoded, then the resulting string is 32 characters long, no more, no less.

So, the answer is 'no', since it depends on how you encode it, but if you know for a fact that it will be hex-encoded, then you can safely use a field length of 32.