I use 17X7''s with 40mm offset and 215's. No complaints on that size. You have to remember one thing about upgrading size, generally the popular sizes have the best prices on tires. Too wide will cost you way too much for just street driven and its only a cobalt, not a corvette. Just saying, don't waste money on a car that isn't a torque monster.
2009 Cobalt LS XFE U74
Mods: K&N Typhoon Intake, Option Cat-Back Exhaust, Cosmo Short shifter, Maxxim Ahead 17X7's, MPx shorty antenna, FE5 Strut/shocks, Tein S Techs, Moog endlinks, Moog spring seaters, ZZP shorty header, LSJ downpipe, LNF front grille