You probably need to find what the rating of the boiler is, either in kW or litres/minute (or both), then estimate what you need from knowing this.
But the largest heat meter in the OEM Shop has DN20 or DN32 fittings (only 16% or 41% of the area, depending on which part of the page you look at!) so I think the pressure drop across this heat meter would be too high. However, downloading the Kamstrup data sheet shows a version with DN50 fittings is available, so you should investigate this. Meanwhile, @TrystanLea, will our heat meter work with to DN50 option of the Kamstrup Multical 403? I assume connections and data format will be the same, what about numerical overflow etc? And if @Oinq needs to order it directly, which options?
After many years in the electrical & electronics industry as a systems/projects/applications engineer, I know only too well that you must always read the data sheet, and anything else that might be in any way relevant. But often more importantly, you must look for the things the data sheet does not tell you - because they don’t want you to know! Then you ask the awkward questions.