got by size.. what looks good to your eye? 600 high is standard look on the manufacturer sites for heat values. I use Purmo they have a decent finish and i ve never had issues with any i ve fitted ( probably 500+)
double panel double convectors are BIG they take up a lot of room on the brackets they stand something like 13/15 cm out into the room but by jove they kick out some heat
i dont give a sausage for can your boiler take it… it makes water hot.. and it will do it all day long.
possibly the issue may be the size of the pipework in the extension, possibly 10mil and plastic which will take longer to heat a rad if the rest of the property is on 15/22mm..
rads are big and white you should notice the heat when you enter a room not the rad..