Which article of the constitution talks about the size of the federal budget?

The Constitution sets limits on the scope of federal powers. It also originally banned income taxes. The only money mentioned in the Constitution is gold or silver. All of these restrict the size of the government and its budget.

Art. I, Sec. 8, cl. 5 explicitly grants Congress the power to coin money. It restricts the individual states to only using gold and silver as legal tender.

Income taxes were never "banned". Before Pollock v. Farmers' Loan & Trust in 1895, federal income taxes were considered an "indirect" tax constitutionally, and thus only had to meet the requirements of Art. I, Sec. 8, cl. 1, which was that taxes must be uniform geographically. Pollock declared federal income taxes a "direct" tax, which thus had to meet further requirements under Art. I, Sec. 2, cl. 3, which called for the receipts of direct taxes to be apportioned proportionally to the population of each individual state.

The Sixteenth Amendment (ratified 18 years after Pollock) lifted the requirement of proportional distribution; it did not change the constitutionality (or unconstitutionality) of federal income taxes.