What is texture size? (making an iphone game and background image too big)?

Main Question (But reading all would be really great for answering my question): What is "texture size?" and if possible how does one change the texture size of a png (If it isn't obvious after answering the first)

Not a programmer. This is for a collaborative project. My background was 9920 by 1280 pixels, at resolution 300. If it helps, I used paint tool sai to make it.

this was the error I received: Skyyy-sheet0.png image (9920*1280) exceeds max texture size (4096)

My background was really long because it was supposed to scroll through the game.

I don't know what texture size is, help would be greatly appreciated. If possible, soon, because tomorrow is our team interim, and we're trying to put in as many assets as possible.

Texture size is the physical dimensions of the image.
Your image is too large (9920 > 4096), and can't be loaded. You'll need to either lazy- load your image or split it into multiple images whose longest dimensions are less than or equal to 4096 texels.
(pixels or texture elements).
Talk to the programmers for the project to see what you need to do. Work may need to be done on the technical side of this project.