probably a regiment or brigade
usually a regiment
Generally, an Army unit commanded by a colonel is called a brigade, regiment, or group. See related links for information about different military organizations and units.
Colonel Leonard Wood, a Regular US Army Officer, commanded the "Rough Riders".
For the British: Colonel Friedrich Baum, who commanded about 800 men, mostly German mercenaries, Lt. Colonel ,Heinrich Breymann, who commanded 642 German mercenaries. For the Revolutionary Army: Militia General John Stark, who commanded 2,000 New England riflemen, Lt. Colonel Seth Warner, who commanded 330 men.
Colonel James Henry Carleton, US Army.
A military officer who commanded a minor division of the Roman army; a captain of a century.
He commanded the Texian Volunteers originally, then shared command with William Travis, a colonel with the regular army. Davy Crockett was also a colonel there, but did not assume real command.
Lieutenant Colonel George Armstrong Custer commanded the US 7th Cavalry Regiment
In the US Army, a full bird refers to a Colonel, which is the rank between Lieutenant Colonel and Brigadier General.
At the beginning of Atlanta Campaign Joseph Johnston commanded the Confederate Army of Tennessee. This Army had been organised on three Corps of infantry and one Corps of cavalry: I Corps under General Hardee, on four division each of them commanded by Gen. Cheatham, Cleburne, W.H.T. Walker and Bate. II Corps under Gen. Hood, on three division each of them commanded by Gen. Hindman, Stevenson and Stewart. III Corps, commanded by Gen: Leonidas Polk on three division, each of them commanded by gen. Loring, French and Cantey. Cavalry corps commanded by Gen. Wheeler, on four division. The total force of Johnston Army was of about 85,000 men, and 187 cannons.
Longstreet and Lee commanded the Confederate army in Gettysburg, while George Meade commanded the Union army. George Pickett led Pickett's charge on the 3rd day, and Colonel Chamberlain held the high ground on Little Round Top.
The question is too vague. A "unit" can be defined as anything from a squad of 8 men to a Regiment or even a Division. Very, very broadly speaking and using the British as a template; 4 men in a Brick 2 or 3 Bricks to a Section (Commanded by a Corporal) 3 Sections to a Platoon (Commanded by a Leiutenant) 3 or more Platoons in a Company (Commanded by a Major) 3 or more Companies in a Battallion (Commanded by a Leiutenant Colonel) 3 or more Battallions in a Brigade (Commanded by a Brigadier) 3 or more Brigades in a Division (Commanded by a 2-3* General) x Divisions in a Corp (Commanded by a 3* General) x Corps go to form an Army Of course, the actual composition of any unit is defined by operational requirements, not to mention the number of personnel and equipment available.