Hi! I'm in exactly the same position, funnily enough, creating a chicken incubating box!
I also reached the conclusion with Cura 4.6.1 that the bed area allowed before slicing issues is 214 x 235mm. I need to print ideally 221 x 217 but I can cut it down to 220 x 217mm and 160mm high
Any ideas or suggestions, I'd love to hear - I've ordered the firmware update kit and stabilizing kit too to help with print quality and bugs...
Mine is an Ender 3, not the Pro version....
Cheers in advance...