Yes—under the simple thin-lens/pinhole-camera approximation, your formula is essentially correct:

real size = image size × distance / focal length

If image size is measured in pixels, then:

real size = (pixel count × pixel pitch) × distance / focal length

So your equation works in principle.

An equivalent practical method is to compute the field of view at that distance, then scale by what fraction of the frame the object occupies. For a 1/1.8" sensor at 25 mm focal length and 700 mm distance, one answer estimated a field of view of about:

• width: 208.26 mm
• height: 138.84 mm

Then if an object spans 50% of the image width, its real width is about 104.13 mm.

Important limitations:

• The object should be parallel to the sensor plane, otherwise perspective distortion affects the result.
• Marked focal length is approximate and usually specified at infinity focus; effective focal length can change with focus distance.
• Lens distortion and other optical imperfections reduce accuracy.

For best accuracy, photograph a reference object of known size at the same distance and setup, then calibrate from that image.