I don’t think there’s a limit but if the dimensions of the image don’t match
that of the project there may be some cropping done by the camera framing
but you should have access to the entirety of the image.
Maybe a screenshot might help to figure out what’s causing it.
Remember that even if the thumbnail looks cropped due to having
a different aspect ratio than the image, you should still be able to
see the entire image when it’s brought into the actual scene in the
Camera view (though you may need to zoom-out if it’s large).