 Its initial size was 32,708 mi2 making it the third largest of the 17 counties created by the first legislature of Idaho Territory in 1863 and early 1864.When the US Congress created Montana Territory on May 26, 1864, it also transferred a portion of Oneida County over to Dakota Territory.Even with the loss of territory, the act left Oneida County as Idaho's largest remaining Idaho county at 20,621 mi2.