When is a region too big to provide meaningful weather information?

I'd like to depict historical averages across the globe for regions of varying sizes. For example: current conditions in Tuscany is xx, Yorkshire is xx, etc. I plan to do this by calculating the historic averages of the region based on the long/lat of the geographic centre of the region.

The problem is as the regions get larger, clearly, the accuracy and value of the forecast diminishes.

Do you have a rule of thumb or suggestion as to the size of a geographic area where a forecast is no longer accurate?

Put another way, is it accurate to provide historic averages for:
Yorkshire (12,000 km2), what about
Tuscany (28,000 km2), surely not for
Baha California (70,000 km2)

I understand this a sliding scale, varies by geography, is a matter use case and that there is not going to be a clear answer. But any thoughts or experience on this would be really helpful.

Thanks!