OP are you looking for (a) size of government (in terms of dollars expended? or employees? or some other factor) in Washington's day vs. late 20th c?

(b) size of economy (as measured by GDP) in the late 18th c vs. the 90s?

(c) size of goverment relative to the economy (e.g., something like government expenditure as a percentage of GDP) in Washington's day vs. the present.

Not a historian, but I'll just note that if you're looking to compare government expenditure in the 18th c with that in the late 20th, you're dealing with apples and oranges. It's not an especially meaningful comparison, I think. Modern government has all sorts of expenses that didn't exist in Washington's time: health care, for instance. Transportation infrastructure. A military capable of global force projection vs. one designed merely to protect against invasion. And so on. What government was in his day versus our own has changed so much.

When measured by GDP, how big was the United States government under George Washington?

Government does not equal economy. This is a mixing of several different ideas.

Are you looking for when the US GDP was the largest or are you trying to define when the US Government was the largest, which is unrelated to GDP?

edit from my comment below:

But really I imagine that OP is looking for something like this. Which comes from here which is Federal Government outlays as a percentage of GDP. This chart goes back to 1791.

Also note the change form GNP to GDP in 1929. Some of an explanation here