I can only answer from my experience, my male guppies max out at about 1 to 1 1/2 inches, without the tail. The females get larger, averaging 2 inches. I don't keep neons, so can't help there.
Neons grow to be about an inch long, maybe a little bit larger if they are the specially bred "jumbo" variety.
Guppies have sexual dimorphism, the male is a lot smaller than the female usually. The male guppy grows to be a little more than an inch long and the female grows to be 2 or 3 inchs long.