Negi in my experience are between scallions and leeks in size. The white part seems to be about as tender as scallions/green onions, not nearly as tough as leeks. They are more pungent than both though, almost as pungent as a regular white bulb onion.
I grew some this year in Portland, Oregon with seeds that I bought on my trip to Japan last year. Started around February inside and then transplanted. They were a good size by June, ready to harvest. I've been pulling them out as needed and since June they have divided into 2-3 small stalks rather than one nice one. I know that Kitazawa has several varieties for sale and there may be some other online seed dealers.
I'd suggest growing some if you can as there are definitely a different product from any other types of onions we have here in the west and are essential for some Japanese dishes.