Should I transplant this seedling yet?

Should transplant yet? And to what size if I should?

I would not. Assuming you have slits at the bottom of your solo cups, I'd wait until i saw some roots. Not sure how many nodes it'd be, i usually top mine. I've read that letting the roots circle the cup, hinders the advancement after transplant. I have noted, after harvest, some root wads definitely have a solo cup sized mass, so sooner is probably better than later. Maybe in a week??
At least you'll be bumped back to the top for another run down the page! Someone should be able to explain it better than i!! Good luck!

Fastest growth = more transplanting using graduated container size. Go to a 2L pot, then 1 gallon/4 litre, then final container. Go ahead and transplant .