You can’t until you have the footage in your hands. The pixels will be determined by 1) the lensing and range on the camera and 2) the video resolution you’re working at (including those pesky D1 frames). A 300′ screen could end up 20 pixels across if they shot it from way-way back.
However given a specific dimension, you can create a rectangle or square or whatever in those proportions. So 37′ by 17′ could equal 3700pixels x 1700pixels. And then hopefully they shoot it dead on.