Flight in most cases is the only practical option to get into Russia (and into the Central Asia), as well as travel long distances internally. Network is well developed and you can get direct flights between most of major cities. Domestic air fares are, though, not much cheaper than in the Europe or the USA.
Luggage regulations are just as everywhere in the world, usually 20 (sometimes 30) kg free, but oversized items may be charged even if you're within the limits (depends mainly from the airline and most of all if that girl at check-in is aware of rules, your negotiation skills are typically appreciated). If you do have some bulky luggage (and you usually do), it is suggested to check-in as early as possible, there's always a chance that plane may get overloaded by the end of the procedure and so they may refuse to accept excess luggage from late passengers.
In many remote areas, like Northern or Eastern Siberia, flying by a small plane or helicopter is the only travel option in existence. Such flights are often unreliable and very much dependent on the weather, so you should have plenty of time and patience.
It is generally possible to hire a helicopter in these remote places to go to an arbitrary point for put-in or take-out, but charter flights are rather expensive. Despite the cost, helicopter may be the only reasonable option to access some remote rivers, for instance, on the Putorana Plateau.