There is a minimum size, this is the size that will maintain the room temperature, but there is really not maximum size, until you reach size of boiler output for a single room, as the larger the radiator the faster it can heat the room. There is a maximum water capacity as either a header tank or some other reservoir needs to cater for the expansion and contraction of the water.
Looking at speed the less water in the system the better, and the less steel, copper or other heat absorbing material the better as well, so likely the fan assisted radiator with a very light matrix holding very little water but massive surface area is the best option, as it can transfer the energy much faster.
Today the whole idea is to only heat a room when required, so if you finish work at 5:30 pm and it takes ½ hour to travel home either using geofencing or simple time, the heating system needs to heat the room you will use on arrival to a comfortable temperature by 6 pm. With a timed response it can have 2 hours, but with geofencing it has ½ hour to heat the room. From cold that will not happen, so it needs some holding temperature, we will call that the eco setting, so the bigger the radiator the lower the eco setting can be.
Using time we can set a sequence, for example kitchen, dinning room, living room, bedroom. When we arrive home first we use kitchen so heat kitchen first, then dinning room so we can have ¼ hour delay before it heats that room, and ½ hour delay before living room and 2 hour delay before bedroom.
But this means in that first ¼ hour we may have a 4 kW radiator and a boiler only able to turn down to 6 kW, since it needs to heat up pipe work etc ¼ hour is not a problem as the dinning room cuts in we are using maybe 8 kW so now no problem, but 1½ hours latter kitchen and dinning room as switched off again and bedroom not yet switched on, so living room needs to sink whole output of boiler.
OK not quite so cut and dried, as rooms not in use are still heated to eco setting to allow quick re-heat when required, so not actually off but reduced output, and below the boilers minimum output it simply cycles off/on. My old house had in living room a 3.5 kW Myson fan assisted radiator, a 4 kW double panel radiator and a 4.5 kW gas fire, there was no real need for geofencing or timed heating with 12 kW the room heated from cold in around 15 to 20 minutes, and the inferred output from the gas fire was there in around 4 minutes so simply switched on when I got home.
This house we are still cold in living room, but can hear the boiler cycling off/on, the boiler can produce 18 kW but the living rooms don't have enough radiator area to sink the 18 kW being produced. With an old fashion system where radiators were set simply using the lock shield valve and all rooms heated up be they require heating or not selecting radiators so all rooms have around the same temperature radiators made some sense, but today the bigger the better.
The programmable TRV can select when and by how much each room is heated as long as the boiler is running, they did cost around £10 for a non wifi/bluetooth head against £6 for non programmable, I got my bluetooth versions for £15 each, and the wifi versions were £37 each, some interconnected versions can cost £50 each, so in the main people use a mixture, to reduce costs, I got them in 3 batches, now use 9 programmable TRV's.
Even if you have no programmable TRV heads yet, as the fuel cost goes up likely you will start using them, so bigger the better for radiator output.
