Think of it this way...a standalone WH often has 30-50K BTU inputs, you have 119K. You can easily heat any of the size tanks you are interested in...I'd get one that can meet your first hour needs. Now, if the tank spec sheet specifies a larger boiler for the first hour rating, you'd have to discount that value since you won't be able to heat the incoming cold water as fast, but it still will - thus, you'd have to start with a larger tank. Often, the spec sheet shows the ratings for various heat inputs so you don't have to interpolate.
If your system is set up with the indirect as a priority zone, while it is calling for heat, the rest of the house won't be being heated, but unless you have a lot of infiltration, that normally isn't a problem. The other option is to treat it as a 'standard' zone, but you'd lose even more first hour ratings when the other zones were also calling for heat. This may not be a big issue, since, unless you have a really big house with lousy windows and no insulation, you probably don't need all that 119K in the first place, so treating it as a standard zone probably won't affect operations much. Still, the norm is to treat it as a priority zone, and it will recover the fastest that way, and then be able to resume heating the rest of the house.