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.
I create solids at different sizes from the comp all the time. If I only need a small object, I make a small solid.
Ahh, that explains it. I work in 1080p 90% of the time so my last 20+ projects are all 1080p. Therefore i never see a solid that i need to "Make Comp Size".
Szalam I normally use the square from the shape tool when i need something smaller and now i wonder what difference would it make is it was just a small solid. I have sorta always believed that there was no difference in the square tool and a solid other than you can do more of certain things with a square.
Shape layers are very different from solid layers.