Well, Osprey claims differently, but most importantly, I never heard of a '2nd battalion 20th maine' or anything of that sort.
Plus, volunteer regiments were differently organized (With a colonel, lieutenant-colonel and a single major) which makes the single battalion formation sound more logical.
Yeah your right volunteers were around 1000 soldiers because most volunteer regiments were a single batalion.
But it really depends on what he is asking. If he wants to know the size of volunteers or regular army.
US Regular regiments were single-battalioned as well. At least, they were in the Mexican-American war, and as far as I know nothing changed between 1848 and 1860 in the organisation.
The regular US army, by the way, was laughably small. Not larger then 13.000 at the start of the war.
The actual size of a regiment is, by the way, hardly representable by its amount of battalions. I've read about Volunteer regiments numbering some 1600 men, while, for example, the 5e de ligne ha two battalions at waterloo, with only 900 men in total. Casualties and desertion made quick work of big formations.
I've personally always liked the 3 battalions, 6 company each-formation of the French. No wonder the Dutch army used the very same system in 1814