What does it mean that photographic works should be 2M in size?

It means that the file capacity of photographic works is 2M.

a 2m photo has two meanings:

First, it refers to the resolution of the photo. A 2m photo is equal to 2 million pixels. Namely, length, width, and 16 and 128.

second, it refers to the size of the photo: 2m means that the photo file is less than 2 trillion. (1 trillion = 124 KB)

Extended data

Image resolution determines image quality. For a picture with the same size, if the image resolution is higher, the more pixels and smaller pixels make up the picture, and the clearer and more vivid the image is. For example, a 1 inch by 1 inch image with 72dpi resolution contains 5184 pixels, while a 1 inch by 1 inch image with 3dpi resolution contains 9 pixels.

image resolution and display resolution are two different concepts: display resolution is used to determine the area size of the displayed image, while image resolution is used to determine the number of pixels that make up an image. For example, on a display screen with a display resolution of 124×768, an image with an image resolution of 32×24 accounts for about 1/12 of the display screen, while an image with an image resolution of 24×3 cannot be completely displayed on this display screen.

For images with the same image resolution, the lower the screen resolution (such as 8×6), the larger the image looks, but fewer items are displayed on the screen; The higher the screen resolution (such as 124×768), the smaller the image looks.