Meme generators are mostly a way to get someone else's watermark onto your picture. Adding the text yourself avoids that.
White text with a heavy dark outline is the convention for a reason. It survives being placed over a bright sky or a dark shadow without becoming invisible, which plain white or plain black text does not.
Size it for a phone. Most people will see your image at a few hundred pixels wide in a feed, so text that looks reasonable on a laptop can be unreadable where it is actually viewed. When in doubt, go larger.
Keep the lines short. Two short lines read instantly, a paragraph does not get read at all.
The captions sit along the top and the bottom, one line each, with up to 60 characters. Because a line cannot wrap, long text is squeezed to fit the width and the letters get small. Short punchy lines read better and let the letters stay large.
Keep the capitals and the outline switched on, which is what makes the lettering readable on a busy picture. White text with a dark outline is the classic choice, and yellow stands out well on a dark photo. Look at the preview at a small size, because most memes are seen on a phone.
No. The image you save is the image you made.
No. The caption tool uses a fixed font, with size, colour, outline and uppercase controls.
No.
Each caption stays on one line, so a long one is squeezed to fit the width. Shorten the words, and the letters can be bigger.