Just guessing based on the size reference of the finger, the radius looks to be in the 18 mm range, so needs measuring like you suggest.Without knowing the coin's diameter, the denomination is not known.
The 4R and the 8R appear the same, only their diameters will differ.
The 4R has a diameter of 34mm 'whilst' (for Cru) the 8R has a diameter of 38mm.
The date range for either coin will be 1761-1771.
Don.....