Answer
The spectral "lines" would be tiny different-colored dots. The overall pattern would be much dimmer.
Work Step by Step
In a spectroscope, the spectral "lines" are different-colored images of the "slit".
This is discussed on page 566.
If large holes were used, the images of the dots might overlap, limiting the spectroscope's resolution. However, a small hole would let through much less light than a slit of equal width, making the spectrum dim and difficult to interpret.
Thus, spectroscopes are designed with slits, instead of holes.
