Just thought I'd add this, to give another option ...
There's a 'Dimensions' panel in the N Properties region on the right of the 3D view. Since the object is aligned to its bounding box you can:
- Enter 1 in the 'Y' field of the 'Dimensions' panel. A new scale will appear in the 'Y' field of the Scale panel.
- Copy-paste the value from the 'Y' scale field to the 'X' scale field.
- CtrlA > Apply the scale of the hexagon.
Not sure if this is what you need.
Enable edge length in the overlay
Add an edge between the two points. You can do this with fill f to make sure it does not split the mesh at all.
Scale the whole object s and hold down the shift to be more precise. Stop when you get to the correct length and then delete the edge you added.