The value in the dialog box is the same as the last one that you used. So, if the last solid that you made was 100x100, then that's what shows up when you make a new one. The Make Comp Size button is very handy then if you want to make a new solid that is the dimensions of the composition.
> In what circumstances would the solid you create be a different size than the comp?
There's a vast number of uses for solid-color layers, and they can be useful at all sizes.
