"Marry, 'tis not so deep as a well, nor so wide as a church-door, but 'tis enough, 'twill serve: ask for me tomorrow, and you shall find me a grave man." Why is this quote significant?

Yes, Mercutio says this to Romeo after being mortally wounded. Romeo is trying to assure Mercutio that his wound cannot be as bad as he believes..... Mercutio replies that it will put him in his grave. Mercutio also places blame on Romeo for stepping in between Tybalt and himself.