Learn Hardcore French: Je suis grand aussi.

Because je in this sentence is referring to a masculine speaker, so the adjective must take the masculine form grand. If a female speaker were describing herself as tall, she would say Je suis grande.

Can "grand" mean both "tall" and "big" in French?

Yes, grand can mean tall when referring to a person’s height and big when describing the size of an object. In the context of someone’s height, grand is best understood as tall.

Is there a difference between "Je suis grand aussi" and "Je suis aussi grand"?

Both sentences are grammatically correct, but Je suis grand aussi places emphasis on also being tall, while Je suis aussi grand can sound more like I am just as tall (especially if comparing yourself to someone else). The word aussi in French can carry both meanings (also / as ... as).

Why do we say "Je suis" instead of another verb when describing size or height?

Être (to be) is the most common verb for expressing characteristics or states of being in French. So to describe your height or size, you use Je suis, literally meaning I am.