Hello: The big water heater manufacturers all have sizing software (usually free) that can be used to determine what you need. The software asks lots of questions about clothes washers, water temperature etc so that it can give you a better guess.
One thought I have is that changing from parallel to series plumbing might get you more usable hot water. You had smaller heaters and apparantly no flow restriction. Your new heaters will likely have even less restriction. When piped in parallel, it’s very important to have even flow through the tanks. A little flow restriction on one tank will force more water through the other, running it out of hot, while the tank with slow flow remains hot. The result is a lot of tepid water. Tanks piped parallel get out of balance with time as rust accumulates inside fittings too. You can sit with the heaters during a period of heavy use and see if one outlet cools long before the other one does. That would verify an imbalance.
It sounds like your hot water system has plenty of heating capacity, but flow trouble… long-haired girls not withstanding 😎