Like the answers to a lot of other poker-related questions, this one is a great big fat "it depends" on all counts.
Starting at the top, the biggest regular cash game in a
us casino is the Big Game at the Bellagio. That plays as high as $4/$8K in limit games, or with a $100K per hand cap in no limit / pot limit. Regular players in this game include Doyle Brunson, Barry Greenstein, Eli Elezra and various others.
Online no-limit games get as high as $500/$1000 blinds with no cap. Phil Ivey, Gus Hansen, Patrik Antonius, Durrr, Urindanger and David Benyamine regularly play at this level. Full Tilt offers limit games up to $2/$4K.
There'll be people making a living from those levels all the way down. The lowest level you can make a living at really depends on your own quality of life values (assuming you're a winning player, obv).
Just for the sake of it, looking at Full Tilt right now (6:30pm Sunday, server time) these are the red pros that are on and the games they're playing:
Phil Ivey is waiting at three $100K NL tables, with nobody to play against. He's also three-tabling $40K PLO.
Gus Hansen: $40K PLO, on one table against Ivey
Ben Roberts: $5KNL
Ryan Daut: four-tabling $400NL
Morris Chestunt: $400NL
Macieck Gracz: $200NL
Louis Velador: two-tabling $1K PLO
Eddy Scharf: $1K PLO
Bruno Stefanelli: $8/$16 LO8
Hal Lubarsky: $1K and $600 NLO8, and $30/$60 Razz (NFI how a blind guy multi-tables, BTW)
Kristy Gazes: $200PLO8
Sigi Stockinger: two-tabling $100 PLO8
Diego Cordovez: $10/$20 Stud8
Esther Rossi: $3/$6 Stud8 and $2/$4 Stud
Roy Winston: $50/$100 and $30/$60 HORSE
