Portrait 4:5 posts take the most space in the feed. Crop your photos to 4:5 or keep the whole picture with clean borders.
1080×1350 px (4:5) for portrait posts, 1080×1080 for square ones.
Yes. 'Fit with border' keeps the whole photo and adds white or black borders.
No. Everything happens inside your browser, on your own device. Your photos are never uploaded, stored or seen by anyone.