How is excretion of metabolic wastes from a cell related to its size?
The critical element here is that building materials that need to be absorbed, or waste materials that need excretion have to cross the cell membrane. The larger the surface of the cell's membrane relative to its volume, the faster the exchange rate. 

The book section you linked in the comments mentions the following on p.14:


  The volume [of a spherical cell] increases by a factor calculated by cubing the radius, [...] the surface area increases by a factor calculated by squaring the radius. [...] This means that a large cell has relatively less surface area to bring in needed materials and to rid the cell of waste [...]. Cells are limited as to the size they can attain [...]. Cells that are larger [...] have modifications that allow them to function efficiently. This is accomplished by shape changes such as from spherical to long and thin, [or] have infoldings or outfolding to increase their surface relative to their volume.