How much extra information is in an RSA public key?
Firstly the length of an RSA key is the length of the modulus and an RSA public key consists of an exponent and a modulus, so there is a couple of bytes there. 

Secondly depending on how you created and/or transmit the key it may be wrapped in a SubjectPublicKeyInfo block. It's defined as (RFC 3280): 

 SubjectPublicKeyInfo ::= SEQUENCE {
   algorithm AlgorithmIdentifier,
   subjectPublicKey BIT STRING }

 AlgorithmIdentifier ::= SEQUENCE {
   algorithm OBJECT IDENTIFIER,
   parameters ANY DEFINED BY algorithm OPTIONAL }


and is basically a way of telling a receiver what algorithm the public key uses and any additional info that may be of use (probably nothing for RSA keys). So there is possibly a few extra bytes there. 

If you're interested in this stuff you can look into ASN1 definitions which describe the structure in which keys and certificates (and many other things) should be passed around. If you want a quick look you can use:

openssl asn1parse -in key.prv -inform pem 


to see what is in the file but in short, depending on the encoding I wouldn't make assumptions on the data size.