I personally think smaller files are better than bigger files. If you have a lot of data that gets queried/modified frequently then it makes sense to spread it over many small files/disk luns to improve IO. Also it could help your SLA with piece meal restores in case of a DR. Large files probably make sense for less frequently queried data that doesnt change and probably belong to a read only filegroup. The data stored in these files also impact the files. If you are going to query the latest data 8 times out of 10 then small or big file size doesnt matter, the key is to store it together since most operations will need to happen within that partition.

Probably a hybrid approach with more files and filegroups for the initial 6 months could work better , have you explored that option ?