Best Of

I would avoid this idea. Some day a new boiler will be needed and someone will decide that reversing the the direction of the water thru the system is easier that running a pipe about 3 feet longer in order to save a few dollars. At that point the oversized lower temperature radiators will become the hotter radiators and the correct size radiator when it was first on the loop, will not get warm enough to heat that room. Baseboard radiators are the only radiators I would recommend using the series loop, and as long as you don't go over 65 to 70 feet of aluminum fins in a loop, then you don't really need to oversize anything based on a lower water temperature. here is one example of wanting to reverse the flow years after system was installed

Another reason not to series loop radiators is TRVs can be added easily.

@Edtheheaterman said

"Keeping things simple is often the best design, then use old school methods to regulate the room temperature as needed. I find that placing the wired thermostat in the coldest room will allow that room to get comfortable, the other rooms that overheat can then be regulated by restricting the water flow to the radiators in the hotter rooms.

If you are designing the system from scratch, you should just do a room by room load calculation and install the proper size radiators in each room. That is the best way to balance the system. One size radiator for each room is not always the best design. Select the radiator to match the room's needs"

And I agree. Put a balancing valve on each rad.