I am not big doesn't work?!
Grand(e) refers to height in humans, and gros(se) to weight or girth. Elle est grande. = She is tall.
However, grand(e) can also mean "great" if it precedes the noun. C'est une grande femme. = She is a great woman.
C'est un grand homme = He is a great man
She is a great woman = C'est une grande dame, c'est une femme remarquable
C'est une grande femme = She is a tall woman