assume we have a box that measures length of it is about one meter. what is percepted size of it for someone looking to it at 100 meter distance?

I'm working on a shooting project that I need to scale 2d hunting goal sprites to make them look from distance.

You are missing [at least] one important number. Let's imagine there is a screen in front of the viewer, at a fixed distance D. Then 1 meter at 100 meters of distance will appear as something at the screen with size 1*(D/100)). You can get that formula by drawing a simple picture of the situation and identifying similar triangles.

You also need to take into account the field of view of your camera. Generally speaking the perceived size follows this relationship

perceivedSize = ratio * actualSize / distance

So you need to determine some ratio, normally determined by the field of view, to base your calculations on. So as n example, if you want a 1 meter to appear to be 1000px at 1 meter distance then your ratio is

1000px = ratio * 1m / 1m
ratio = 1000px*m/m

So at 100m the target would be

percievedSize = 1000px * 1m / 100m = 10px;

1*(D/100)).

i think formula will lok like : 1/(100-D) because for example if distance of screen from eye is 10 meter. so it means if you want goal to be look like in 100 meter so goal have to be look like 90 meter distant. am I right?

1*(D/100)).

i think formula will lok like : 1/(100-D) because for example if distance of screen from eye is 10 meter. so it means if you want goal to be look like in 100 meter so goal have to be look like 90 meter distant. am I right?

+----------+    ^
|         /     |
|        /      |
|       /       |
|      /        |
|     /        100
|    /          |
+---+--- ^      |
|  /     |      |
| /      D      |
|/       |      |
*        v      v

Picture the eye at the `*' in that diagram. The screen is at a distance D and the object is at a distance 100. There are two similar triangles in that diagram, so you can see that

apparent_size / D = real_size / 100

So apparent_size = real_size * D / 100