It is a simple matter to estimate the length of an object from a picture, given you know the focal length and distance.

You measure the ratio of the length of the object to the picture's side. For example, if the picture is 3' on a 4R print, then the ratio of the item-picture is 3/6 or 0.5.

From the focal length, you either look up, or calculate, the horizontal angle of view (bcoz the long side of the picture was used in calculating the ratio). For example, a 50mm lens would have an horz angle of view of 39.6 deg. From the ratio calculated above, the object would take up an angle of (approx) 0.5 x 39.6, which gives ~20 deg. (You can calculate the angle more exactly on paper, but you just want an estimate, right?)

Let's say the distance on the scale is 7m. With the angle of view and the focussing distance, simple trigo would give the object length to be 1.2m (use tangent: x/7 = tan(20/2) ).

If you need more precise 'estimation', note:

- most shops crop the picture a bit when they print for negatives. Use the negative to measure, not the print.

- the distance scale is terribly inaccurate for longer distances.

- lens distortion would shorten the length of the item if it is near the edge.

Hope this helps.