*(Using Blender 3.6.8)*

### Approach ###

Use two cubes as guide, combined with <i>Snap to Vertex</i> activated.

### Setup ###

Using the middle top icons of the <i>3D Viewport</i>:
- Set <i>Transform Pivot Point</i> to <i>Individual Origins</i>.
- Set <i>Snap to</i> to <i>Vertex</i>, and set <i>Affect</i> to <i>Move</i> and <i>Scale</i>.

### Step 1 ###

[![Step1 Position reference cube][1]][1]

<b>1. </b>Set the cursor location to a first reference position (e.g. the centre of Suzanne right eye).<br>
<b>2. </b>Displace the object to scale origin to the cursor location (menu <i>Object/Set Origin/Origin to 3D Cursor</i>).<br>
<b>3. </b>Add a cube (menu <i>Add/Mesh/Cube</i>).<br>
<b>4. </b>Scale it such that one of its dimensions (e.g. X) matches the specific sought value (menu <i>Object/Transform/Scale</i>).<br>
<b>5. </b>Apply the cube transformations (menu <i>Object/Apply/Scale</i>).<br>
<b>6. </b>Set the cursor location to one corner of the cube.<br>
<b>7. </b>Displace the cube origin to the cursor location.<br>
<b>8. </b>Select the object to scale and move the cursor to its origin (<kbd>Shift S</kbd> <kbd>2</kbd> <i>Cursor to Selected</i>).<br>
<b>9. </b>Select the cube and move its origin to the cursor location (<kbd>Shift S</kbd> <kbd>8</kbd> <i>Selection to Cursor</i>).

At this stage, the object to scale and the cube are sharing the same origin, and one side of the cube matches the sought length.

### Step 2 ###

[![Step2 Position guide][2]][2]

<b>1. </b>Select the cube and duplicate it without displacement (menu <i>Object/Duplicate Objects</i>).<br>
<b>2. </b>Set the cursor location to the corner opposite to the origin of the copied cube.<br>
<b>3. </b>Scale the copied cube moving the mouse such that the cursor snaps to the second reference position (e.g. the centre of Suzanne left eye).<br>
<b>4. </b>Apply the copied cube transformations.

At this stage, one side of the copied cube matches the original length of the object to scale.

### Step 3 ###

[![Step3 Scale guide to match reference cube][3]][3]

<b>1. </b>Add a <i>Copy Scale</i> constraint to the object to scale.<br>
<b>2. </b>Choose the copied cube as <i>Target</i>.<br>
<b>3. </b>Select the copied cube.<br>
<b>4. </b>Set the cursor location to the corner opposite to the origin of the copied cube.<br>
<b>5. </b>Scale the copied cube moving the mouse such that the cursor snaps to the right corner of the original cube.<br>
<b>6. </b>Select the object to scale and <i>Apply</i> the constraint to freeze the scale.

**Resources:** [<img src="https://blend-exchange.com/embedImage.png?bid=65EwxY80" />](https://blend-exchange.com/b/65EwxY80/)


  [1]: https://i.sstatic.net/5GKnDdHO.png
  [2]: https://i.sstatic.net/CbIUJVOr.png
  [3]: https://i.sstatic.net/QsuipTyn.png