What Size Screen?

[Tigress22]

I have a Hitachi pj tx100 projector and I want to finally get a screen instead of projecting on a white wall. What information would I need to know to make sure I get the right size screen? The room I will be setting it up in is 11ft wide and 8ft long. So it’s not that big also the wall I will be projecting on has heating ducts running along the ceiling so the image cannot go right to the top of the room so from the bottom of the heat duct to the floor is 6ft 3in. Any information on what size and specs would be great.