How can I get width that multiple selected items?

That's right Carlos

@kimDino8,

my script in Carlos link creates a green rectangle around the selected items. If you don't need this rectangle than you can remove this lines:

var newRGBColor = new RGBColor();
newRGBColor.red = 0;
newRGBColor.green = 255;
newRGBColor.blue = 0;

and replace this:

var aRectangle = aDoc.pathItems.rectangle(vBounds_Ob, vBounds_Li, vBounds_Re - vBounds_Li, vBounds_Ob - vBounds_Un);
aRectangle.strokeColor = newRGBColor;
aRectangle.strokeWidth= 2;
aRectangle.filled = false;
aRectangle.stroked = true;
aDoc.selection = null;

with this:

var mmFactor = 2.83464567;
alert("width = "+ (vBounds_Re - vBounds_Li)/mmFactor); 
alert("height = "+ (vBounds_Ob - vBounds_Un)/mmFactor);

Further you only have to do: create a function for rounding of the result.