How big should an embedded image be? Give your opinion.

200/10 testing full-speed at both of those. Of course it also tests full-speed on fast.com and speedtest.net shrug

Honestly, what matters the most is actually how much you can compress down your image using various codecs.

Something like uncompressed PNGs will not do all that well, but compressed with something like ECT will squeeze some nice efficiency out of an image. This is lossless compression.

Then you have WebP, which has 3 modes: lossless, near-lossless, and lossy. The lossless mode is currently the best all around lossless image codec, near-lossless is your best bet for optimal quality at a nicely reduced file size vs lossless, and lossy is lossy of course.

You then have JPEG too, which can be compressed nicely using something like Mass Image Compressor(based on libjpeg turbo) or Pingo(all around closed source GUI based image compressor).