You have to ask yourself how long a typical shower will be, and that may include adding back-to-back shower times. Discounting the WH's ability to reheat water, you can use about 75-80% of the capacity. Now, depending on how hot you set it, your shower doesn't use full hot, so figure out how much actual hot you need. Then, look at recovery rates and first hour capacity. As soon as the WH detects the temp dropping, it will start up the burner, and that will extend it some beyond the 75-80% (which is what gives you the first hour capacity). Tank burner output varies quite a bit, so you can get quite a bit out of a smaller tank if you want. You can work out what you need from that, then try to match it up with a model and brand.