Poland is the usual choice divided up between Russia, Prussia and Austria. The problem is what after that, you now have only Turkey and Sweden as potential enemies. Decide what you wish to do early on and build the army in that area. Don't make it too obvious though, i.e. don't gather two hundred battalions in a barracks recently built on the Turkish border. Lumber and food are the two most important goods to Russia.
Lumber is used for both re-populating areas of low density and also building and repairing ships.
Russia uses it in quantities far greater than other counties. Food is essential. Russia has a massive potential for food production but uses it in huge quantities. A figure of 2 million will be attained before the end of 1809 if you invade any country or put large numbers back into the population. Don't let them starve, it would bankrupt the economy.