Common used bolts size for bicycle (Titanium):

M5*9  ：for front Derailleur, Rear Derailleur etc
M5*10 ：for fixing bottle cage , disc-Brake , Rear Derailleur etc
M5*12 ：for fixing bottle cage , disc-Brake etc
M5*18 ：for fixing front Derailleur , fixing seat post, Clamps Stems etc
M5*20 ：for Brake lever , Derailleur etc
M5*40 ：for Seat post etc
M6*10 ：for V brake etc
M6*12 ：for V brake etc
M6*16 ：for Thumb Shifter, Brake Lever etc
M6*19 ：for Crank Only, Stem
M6*20 ：for Brake lever ,Derailleur , Stems etc
M6*25 ：for V brake, handlebar etc
M6*30 ：for seat post, Headsets etc
M6*35 ：for seat post etc
M6*40 ：for Stems, seat post etc
M8*18 ：for head tube etc
M8    ：for V brake, Forks etc
M10   ：for V brake, Rear Forks, fold bike etc