Is this from a user perspective, or are you an admin and would like to add this customization to your site?
An admin could add this CSS to a theme, which would force the composer to always occupy the full-page height.
body:not(.ios-safari-composer-hacks) #reply-control.open {
--min-height: 100vh;
}
This also prevents it from being resized at all, so other people on your site that don’t want this might find it annoying.