If you mean to ask for a sentence containing the word Backdrop:

He stood motionless, his coat, red, peircing the misty backdrop of the night.

If this is not what you are after, my apologies

Synonyms for backdrop: Scenery or scrim

Mostly backdrop

A backdrop is a decorated cloth hung at the back of a stage, or the setting or background of some historical event.

Yes, the size of a photo backdrop matters, as it must encompass the entire background of the picture. If the backdrop is too small, the background in the picture will be a different colour.