No British King's Colour has ever had a green stripe. They may well have some colour of the regimental insignia in the centre.
I think what you are seeing is a highland ensign with the green being the regimental plaid.
With regard to the size, Barrel's (later 4th King's Own) Regimental colour in 1746 was approximately 6' 6'' in length and 6' 2'' on the staff.
It's probable that infantry colours in 1759 were the same size, but I wouldn't like to assume it.
Cheers,
Andrew