Canvas Size

You cannot calculate it using Canvas Size as shown, because to work out the file size, you must know how the pixel dimensions of the image, so Width and Height must be set to pixels.

I determined the specs using Image Size. If a document of those dimensions in cm is set to 72 ppi, then it will have a 51.3MB file size if it is set to 8 bits per channel.

The math is not specific to Photoshop, it works in all digital image editing in any application. If compression and layers are not involved, the file size is always:

Number of pixels wide

times

Number of pixels tall

times

Bits per channel

times

Number of channels

If a document is

182.88cm wide

121.92cm tall

set to

72 ppi

then it is

5184 x 3456 px

which equals

143,327,232 bits

times 3 RGB channels, which equals

429,981,696 bits, which equals 53,747,712 bytes,

or around 54MB

…which is close to the file size shown at the top of your Canvas Size screen shot.

(I haven’t worked out why it’s more than your 51.3MB unless your actual pixel dimensions are different than my educated guess of 5184 x 3456 px)