yeah, the player chooses. They're a blank slate the player can project themselves onto, thus they don't have a cannon personality.

It would just be a blank slate since Frisk doesn't have a personality in-game. Sorry

a giantess is just a female giant so in my opinion 20 feet tall or taller

Mate, I'ma be honest, I just don't know anything about giantess stuff, and tbh, I don't have enough to make a high fidelity model anyway. Also, they're like 11 years old, and people on this site are unhinged.

because that's how I think of Frisk since they don't canonically have a gender

but I specified pacifist so a nice caring person that wants to be everyone's friend even if they try to kill them but also I'd want it to be a female Frisk

Toriel 

can you make a giantess pacifist Frisk please?