DBGrid HELP : Field Length limited ????

Hi

i've got a pb....

i'm connecting to a mysql database througth ODBC and i have fields of type
VARCHAR(255) ....

When i try to fill a field in the dbgrid, i cannot write more carachters
than the longuest word in this field (from others records)....

What can i do ??

THX

Jerome

PS:Sorry for my english, im french