Can public key help attackers to break RSA encryption faster? I have two, closely related questions:


Can a really good attacker (hacker) use the public key for his advantage and break RSA encryption faster? My point is that in RSA "communication" public key is moving from server to client… If attacker will get this key and also encrypted data will be there any advantages or it is not important if attacker has public key?
If, for example, there are five users and each user has public key but with different size (2048, 2100, 2120,2250,2310bits etc.) If attacker will break one of these key all other keys are also immediatelly know for this attacker or he must start whole process of breaking again? Is there some advantages to have each RSA key with different bit size?
Can really good attacker(hacker) use public key for his advantage and break RSA encryption faster?


No; we always assume that the attacker knows the public key (that's what "public" means, after all).


  Is there some advantage to have each RSA key with different bit size?


No; the most efficient approach known to attack an RSA key is to use the Number Field Sieve algorithm; which that approach, the attacker picks one RSA key, and attacks it.  It doesn't matter if the keys he selects from has different sizes; he's likely to pick the smallest and/or the one that protects data of the most value.