Proprietary file formats, (often ending in  *.NEF, *.CR2, *HEIC, *RAW, etc.), must be opened by an image application such as Photoshop, Lightroom, CameraRaw, or CaptureOne in order to view the size information for the image.
For standard file types; such as JPEG or TIF -
In Finder on Mac: Right-click the filename, choose Get Info. In the More Info subsection, look for “Dimensions”.
In File Explorer on Windows: Right-click the filename, choose Properties.  On the Details tab, scroll down to the Image section to “Dimensions”.
Example:
An image that is 2500px X 3200px will print well up to an 8”x10”
2500/300 = 8.33      3200/300 = 10.66