Feliway did help my cat but it gets expensive and it doesn’t work for everyone. Mine wasn’t intentionally marking anything but had an infection on top of anxiety from moving house and was mildly incontinent for a time. Aside from treating the infection itself, keeping him hydrated (new water fountain and more wet food rather than dry) and being extremely meticulous with cleaning the litter tray helped the most. I also have him on a special diet for it as a precaution, but it’s not cheap…I probably spend more on his food than my own lol but he seemed really miserable when he was dealing with the urinary issues and I don’t want him to face it again. I hope your kitty feels better soon!

Based on my observations of my own cats over the years, the smaller the box the better. But I think any box is good