heres the deal... I am not 100% sure what size my PJ will be, it should be between 90" and 100" the figures I have are rough estimates and could be off a little bit...

I am thinking about making the screen around 100" and then sizing it down by painting the unused area with flat black paint...

I will be using screen material from a Da-lite model B 72'' x 96'' screen

or should I just paint the wall? I am already going to paint the wall flat black to help with contrast

Since you are going to cannibalize a Da-Lite screen, I'd recommend you wait until your projector is finished so that you can place it at the right distance, project an image, and then cut the screen to that size.