For "in size" in this context think it as being in a category. For example our country tripled -- tripled in what? It tripled in the category of size. Or it tripled in the category of GDP (Our economy tripled in GDP). Or it tripled in the category: number of consumers. (Our economy tripled in number of consumers.)

Jackson makes a good point. However, the implication is that the growth is the “dollar”/currency value of the economic output tripled. So, the “in size” is redundant. “The economy tripled.” is fine on its own.

The economy grew three times larger than it was before.