The size of the standing army is set by statute.
Not even Lincoln tried to claim his actions were legal. Lincoln said, "These measures, whether strictly legal or not, were ventured upon, under what appeared to be a popular demand, and a public necessity; trusting, then as now, that Congress would readily ratify them."
Lincoln also said, "It is believed that nothing has been done beyond the constitutional competency of Congress."
Lincoln was not Congress. He possessed no competency of Congress and no authority to act as Congress.
Lincoln did not defend his actions on any legal basis, but claimed "popular demand" and "public necessity."
But Lincoln did ask Congress to legalize his actions ex post facto [which is unconstitutional], and even then they failed to legalize all of them. Lincoln had earlier stated that invasion of a state under any pretext was illegal, and had also said this 23 Jul 1856 in Galena,
"We, the majority, being able constitutionally to do all that we purpose, would have no desire to dissolve the Union. Do you say that such restriction of slavery would be unconstitutional and that some of the States would not submit to its enforcement? I grant you that an unconstitutional act is not a law; but I do not ask, and will not take your construction of the Constitution. The Supreme Court of the United States is the tribunal to decide such questions, and we will submit to its decisions; and if you do also, there will be an end of the matter. Will you? If not, who are the disunionists, you or we? We, the majority, would not strive to dissolve the Union; and if any attempt is made it must be by you, who so loudly stigmatize us as disunionists. But the Union, in any event, won't be dissolved. We don't want to dissolve it, and if you attempt it, we won't let you. With the purse and sword, the army and navy and treasury in our hands and at our command, you couldn't do it. This Government would be very weak, indeed, if a majority, with a disciplined army and navy, and a well-filled treasury, could not preserve itself, when attacked by an unarmed, undisciplined, unorganized minority.'All this talk about the dissolution of the Union is humbug---nothing but folly. We "WON'T" dissolve the Union, and you "SHAN'T".'" [italics in original, emphasis mine]
Abraham Lincoln, "Speech at Galena, Illinois", Collected Works Of Abraham Lincoln, Roy P. Basler, ed., New Brunswick, NJ: Rutgers University Press, 1953, Vol. II, pp. 354-355