If you want to make your pin in the Pinterest recommended size, you’d need to resize your post on Canva into 1000x1500px.
However, if you just want to quickly forward your post to Pinterest to reach a wider audience, both of the ways described work. Yet, when you share your Instagram post to Pinterest directly, Pinterest automatically picks the pin description from Instagram, whereas when you save your Pinterest to your camera first and then create a new pin, you can edit the pin title and description.