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How large was the Grand Duchy of Lithuania under Vytautas the Great?

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Map showing Poland's territory in 1634 under the reign of Wladyslaw IV Vasa, shaded in pink and spanning from the Baltic Sea in the north to near Kyiv in the southeast. Major present-day cities such as Warsaw, Krakow, Gdansk, Vilnius, Minsk, Lwow (Lviv), Pinsk, and Kyiv are marked within the territory, with surrounding cities like Moscow, Riga, Vienna, and Budapest shown for reference. — Polish-Lithuanian Commonwealth Poland's territory in 1634, during the reign of Władysław IV Vasa.

Under the rule of Vytautas the Great, the Grand Duchy of Lithuania became the most powerful state in eastern Europe. During his tenure as viceroy and supreme ruler, the state expanded its borders to the Ugra and Oka rivers in the east.1

Following the decisive Battle of Grunwald in 1410, where Vytautas led Lithuanian forces alongside Poland to defeat the Teutonic Knights, the Grand Duchy gained control of the northwestern territory of Samogitia, a gain that was formally confirmed in 1422. By this period, the state’s influence extended significantly, as Vytautas assumed a dominant role in both Tatar and Russian political affairs.1