Your flag will usually be resized to fit within 321x213 pixels. Both height and width must fit, so if, for example, your flag is perfectly square, it will be resized to 213x213. This process also converts animated GIFs into static images.
Exceptions: You can have a bigger flag than usual if its filesize is small enough: If it's under 50kB and the image fits within either 428x284 pixels or 107x426 pixels, it won't be resized.
The flag is under 50kb and within 428x284 pixels, yet it is getting compressed. Is there anything I can do about this?