My SO bought a small one (6kg) and I wanted fruit for some reason so I tested myself again.
I ate 800g and couldn’t not eat more. So it’s still 1 kg per occasion for me.
I don’t even like watermelon! And it wasn’t good. Too sugary and I got too full. No other problems as I am not that type. I can get sugar poisoned but that takes more after almost carnivore days 
I am almost invincible after them.
Won’t touch it this year again for sure but I really hope never…
And I am super good at tiny amounts regarding most things, it’s just an exception for me. It’s basically water, just sugary and crunchy.
I wanted the other kind of melon but it was expensive 
On the other hand, I was perfectly happy with 10g banana when I was a beginner ketoer. Now I am happy with zero.