You should get something that expands when urine touches it that way you can keep an eye on something like that. I don’t know about you, but I wouldn’t want my buns peeing on my floor and not catching it on time to pick it up. It will start to get a strong ammonia smell in your home.
We use feline pine cat litter and it starts off as pellets but when it gets wet they crumble so you can easily tell when they pee in their box
Also, the litterbox habits should get easier after a neuter and after he has time to allow the hormones to dissipate (can take 4-6 weeks post neuter)
Also, you shouldn’t have to go much smaller than a 4×4. What you need to do is instead of making his house smaller, is to limit the amount of time he is allowed free roam. And when he is out running around watch him like a hawk that way you can catch him when he makes an accident.
So limit his outside time, but don’t make his house any smaller 