If you have the vertical space where you can install one, with the showers, a waste heat recovery system would mitigate the need for a super large WH. There's a lot of discussions here, so use the search function. With a lot of tub baths, there's usually some time inbetween, so there, the recovery rate is less important, and the waste heat recovery system won't work. One way to make any WH appear larger is to run them hotter. Where I live, they require a tempering valve, regardless of what temp you run the WH. Mine's fairly old, and still seems to work. In a tub/shower, the modern valves all have a high limit adjustment you should use and check periodically. I prefer a thermostatically controlled valve in the shower/tub. There, it doesn't really mater what the incoming hot is, it adjusts itself to your preference.