Foreword
This a rather large question, and it's going to be hard to really explain in detail how the different sizing routines work and play together. That's why the Engineering Reference has such a huge section treating about sizing.
I'll do my best to get to the point and you'll have to forgive any gross simplification.
The first of such simplifications, is that I'll take out the WaterHeater:Mixed from the problem, I'm assuming it doesn't have a built in heater and is just served by the PlantComponent:TemperatureSource[1]
Conclusion
I'm writing the conclusion here because the section where I explain the sizing is lengthy
What you need to really remember: to size your hot water coil, the program actually gets user inputs defined in several objects and uses several sizing routines. It will use the design temperatures (and air flow rates in some cases) defined in the Sizing:Zone object and the resulting calculated laods, as well as the plant delta T you specify in Sizing:Plant.
It's up to you to determine whether you do need to feed information for the sizing routines to work properly.
My two cents is that if in your application the zone design temps aren't changing, and if you intend to keep the same constant delta T in the plant loop, you shouldn't need to feed new info for co-simulation.
If you do need to change it for the sizing routines, I'm sure there's a way (post a new question though).
Sizing Manager
For the sizing, it's done on the design days indeed.
It's going to size stuff in this order (that's what I understand at least): Zone, System (airloop), Component, and Plant. Important: please read more in the Sizing Manager section of the engineering reference.
(FYI, it's also going to start by looking at the global Sizing:Parameters)
Zone Sizing
First, it's going to size your zones, based on the Sizing:Zone (I/O) object, in which you enter the supply air temperature, humidity ratio, and potentially air flow rate (or tell it to calculate the air flow rate based on design days). That'll give you the heating and cooling loads.
You have no airloop here, so System is skipped.
Component sizing
Second, it's going to look at your components, in your case the ZoneHVAC:Baseboard:Convective:Water, and try to size the heating coil. The relevant section of the engineering reference is Coil:Heating:Water Sizing > Zone Coils
Get input from Zone Sizing
First, it gets a few input stored in the Zone Sizing routine in Zone Design Data Array (read for variable definition)
It gets the zone heating coil design air inlet temperature [C]:
$$T_{in,air} = DesHeatCoilInTemp_{zone}$$
This is calculated by checking the zone temp at sizing (from the Tstat), and the potentially Outside Air Fraction and temperature (at the design day).
And the zone design heating ... (more)
