Basically, no, not for practical purposes. The larger the radiators the lower you can get your flow temperature and therefore the more efficiently your heat pump will run.
A downside would be where some radiators are much bigger in some rooms than others (relative to the room heat loss). In that case it becomes hard to balance the system, the rooms with the big radiators get too hot and those with smaller radiators struggle to get up to temperature. You can manage this by balancing the radiators using lock shields or TRVs but that undermines the benefits of the larger radiators. If I were advising somebody looking at new radiators, it would be to make sure that the rooms they are most concerned about being warm most of the time (living room, study, kitchen?) have the largest radiators relative to room size. This is what will determine the lowest flow temp that the system can be run at.
On the subject of radiators, in the cold patch a couple of weeks ago (daytime high of -2C here) I was experimenting with very low flow temperatures <35C. The results of that doesn’t tally with my predictions of the heat that the radiators should be able to put into the rooms. My calculations, using Stelrad’s correction factors, gives a theoretical total radiator output of <2kW at that sort of flow temperature. However the heatpump was steady state running at nearer 3kW heat production. Assuming the produced heat largely goes into the radiators, this must mean that the calculated heat outputs of the radiators are quite a way off.
Has anyone else found the same result? (or a different one?) I haven’t done any experiments at higher flower temps. If accepted formulae for radiator outputs for a given flow temperature are also underestimates then this would be a further factor in Tristan’s analysis that “calculated” flow temperatures are far higher than they need to be.