Beautiful painting !!
A new flat from Vladimir Nuzhdin, small picture from his site, declared 7cmx9.5cm as plate, calculated to be around 75mm size (feet to eyes level).
(https://i87.servimg.com/u/f87/19/31/59/25/alexan10.jpg)
Based on a painting of Vladimir Lukič Borovikovskij (1727-1825) "Portrait of Grand Duke Alexander Pavlovich of Russia" in 1800, one year before his coronation.