There are two factors which affect capacity. One is the tank size, and the other is the burner rating.

My guess without getting into a lot of detailed calculation is that you would be best to go with a 50 gallon tank, and factor a reasonable cost for water into the rent charge. Probably hard to recover the cost of a separate heater.

Even when you decide on a 50 gallon size, there are probably a couple of ratings to choose from. To be safe you might want to go with the larger if it is a reasonable cost differential. A 50 gallon model may be available with 40,000 or 60,000 btu input. Recovery will be about 50% faster with the larger.

We just bought a GSW (same as John Wood, AO Smith) 50 gallon 40,000 btu with a powered vent. For three of us (low efficiency washer, three showers, dishwasher), we never run out of water. The only down side is that it is noisy. If you go powered you may want to investigate the noise rating.

Thanks they will be atmospheric vent so they should not make noise.

The 50 gallon from lowes has a 40,000 btu burner
The 65 gal has a 65,000 btu burner
The 55 gal high output had an 80,000 Btu burner

The thing that makes me a little afraid to go with a single standard 50 gal is that we have had a tenant with a low efficient washer for a short time and before she got evicted for non payment she washed up all her and her kids clothes and probably her friends too. Then I did run out of hot water. Ok ow that was not the normal and more then average demand with two washers.
But I want there to be enough

I see there is a Bradford white 75 gal with a 76,000 btu burner
Would that be better or too much?

Check the 'first hour recovery' rating of the heater.

The only downside of a larger tank is more space required, and initial cost differential. Usually the efficiency rating is the same. Bigger burner is usually just initial cost, and efficiency remains unchanged. I don't think there is any downside other than cost and space for going too big. In heating seasons the losses from a heater tank are going to be offset by reduced heating, so no real loss.

All the ones you listed are going to put out substantially more hot water than the Lowes 50 gallon, 40,000 BTU model. Based on getting by on a 40 gallon before you should have more than enough. I would just select between them based on efficiency and cost. A 50 gallon, 60,000 BTU unit would likely work too if you can find one. Low flow shower heads are often a good investment to reduce hot water demand. You can get good ones for only a few bucks.

My theory on hot water heaters is that they will all be very long life if you change the anode rod regularly. We got 32 years out of a basic 40 gallon John Wood (AO Smith). Anode rod was changed twice but need varies depending on how aggressive your water is.

Ok

The 65 gallon I referenced has a first hour out put of 127 gal recovery rate of 69 gph. About 50% bigger than current unit
The 75 gallon has first hour of 138 gal and recovery rate of 82 gph near 100% bigger than my 40 gallon

The 55 gal high output has first hour of 200 gallons. Recovery of 86 gph not much bigger than the lowes 50 gallon but much bigger burner

I looked on lowes and homedepot sites all the 50 gallons are 40,000 btu

I like the Bradford white units they have brass valves and good quality controls. So it seems unless I can find a 60,000 btu in a big box I am going to get a liscened co worker to get me one of the Bradford white. Likely the 65 gallon but still waiting for pricing

I do have low flow showers. I think they are 1.5 or 2.0 gpm flow in both baths mine and the renters. However they have a claw foot tub with shower kit and I am sure the kids take a tubby bath. I also want to be ready for a second clothes washer

If each unit has its own gas meter, then I'd think separating the hot water service would be the best approach. But maybe the needed replumbing would be prohibitively difficult or expensive, given the original single home layout

If gas service isn't separate, unless you want to pay for that too, then your choices are limited.

Having one water heater for two separate units seems a bit half-a$$ed to me.

Yes each has its own gas meter There are actualy 3 gas meters. On for the space and water heating that is shared by one appliance for each duty and the other two for each apartment that both currently only have a stove on each. To get them to each pay for the water heating I would have to have the gas line taken off the shared house meter and set the new units up on each apartments meter. Then plumb the water from them to the feeds for the fixtures in each unit. To me that would be the best way and it would be one less small thing as an owner I was paying for and had to add in to the rent. It would also give both a more consistent supply not effected as much by what was being used in the other apartment.

I do need to look in to that option more and the costs of making that change. What sizes would I use then If I were going to use two heaters?

I was thinking if I went that way I would get a 30 gal for me and a 40 for the larger apartment or just 2 40 gallon