The key to these is making sure all of the wire ends are smoothed and that you use a closed ring so that the metal does not catch on the yarn. I made these with what I happened to have on hand, though if I were going to make these all the time I would invest in several sizes of closed jump rings.
These markers are for her US size 7 needles, which are 4.5 mm, though they'd fit up to an 8mm needle. I can't knit to save my life so I have no experience with what the size of the loop might mean while being used.
I also experimented with hand wrapping the wire on the top to be just slightly larger than the needles and I think they'd work just fine, but I didn't like that they weren't as uniform. According to what I've seen on Etsy considering these are made with lampwork beads and Swarovski crystals, they would probably sell for $10 or $12 for the set. The Minlaw gets these for free and I'm hoping she'll show them off in her knitting class!