Here is a script that will resize SELECTED layers to 6" in height.

#target photoshop
function main(){
if(!documents.length) return;
var selectedLayers = getSelectedLayersIdx();
for (var z in selectedLayers){
selectLayerByIndex(Number(selectedLayers[z]));
resizeLayer();
    }
}
main();
function resizeLayer(){
var startRulerUnits = app.preferences.rulerUnits;
app.preferences.rulerUnits = Units.PIXELS;
var doc = activeDocument;
var res= doc.resolution;
var LB = activeDocument.activeLayer.bounds;
var Height = LB[3].value - LB[1].value;
var onePix = 100/Height;
var newSize = onePix * (res * 6);
doc.activeLayer.resize( newSize , newSize, AnchorPosition.MIDDLECENTER); 
app.preferences.rulerUnits = startRulerUnits;
}
function selectLayerByIndex(index,add){ 
 add = (add == undefined)  ? add = false : add;
 var ref = new ActionReference();
    ref.putIndex(charIDToTypeID("Lyr "), index);
    var desc = new ActionDescriptor();
    desc.putReference(charIDToTypeID("null"), ref );
       if(add) desc.putEnumerated( stringIDToTypeID( "selectionModifier" ), stringIDToTypeID( "selectionModifierType" ), stringIDToTypeID( "addToSelection" ) ); 
      desc.putBoolean( charIDToTypeID( "MkVs" ), false ); 
   try{
    executeAction(charIDToTypeID("slct"), desc, DialogModes.NO );
}catch(e){}
};
function getSelectedLayersIdx(){ 
      var selectedLayers = new Array; 
      var ref = new ActionReference(); 
      ref.putEnumerated( charIDToTypeID("Dcmn"), charIDToTypeID("Ordn"), charIDToTypeID("Trgt") ); 
      var desc = executeActionGet(ref); 
      if( desc.hasKey( stringIDToTypeID( 'targetLayers' ) ) ){ 
         desc = desc.getList( stringIDToTypeID( 'targetLayers' )); 
          var c = desc.count 
          var selectedLayers = new Array(); 
          for(var i=0;i<c;i++){ 
            try{ 
               activeDocument.backgroundLayer; 
               selectedLayers.push(  desc.getReference( i ).getIndex() ); 
            }catch(e){ 
               selectedLayers.push(  desc.getReference( i ).getIndex()+1 ); 
            } 
          } 
       }else{ 
         var ref = new ActionReference(); 
         ref.putProperty( charIDToTypeID("Prpr") , charIDToTypeID( "ItmI" )); 
         ref.putEnumerated( charIDToTypeID("Lyr "), charIDToTypeID("Ordn"), charIDToTypeID("Trgt") ); 
         try{ 
            activeDocument.backgroundLayer; 
            selectedLayers.push( executeActionGet(ref).getInteger(charIDToTypeID( "ItmI" ))-1); 
         }catch(e){ 
            selectedLayers.push( executeActionGet(ref).getInteger(charIDToTypeID( "ItmI" ))); 
         } 
      } 
      return selectedLayers; 
};


 

Hey Paul  great script I'm already making use of it  thanks! One question, Is there a way to make also fit 6" width I'm working on square documets with 7 layers each.  4 of them have to be sresized in width and 3 of them in height. Can this script be modified in some way?

Hey Paul thanks for  your answer, however this is  more of a problem because I have to make the input manualy and one layer at a time. I'm looking  for something like a fit image to 6" but aplied to selected layer" I think I'm not explaining my goal I'm sending you a screenshot of the result of your great script so I can explain myself better.

Ypur script is working perfectly on layers 3, 5, 6 and 7

Thanks again!

Thanks Paul I always wanted to know how to process selected layers well.  

The OP may want to catch the transform resizeLayer error that will happen if the background layer is selected. Put out a  message or just ignore the error.

function resizeLayer(){
          
          
          try{
                    doc.activeLayer.resize( newSize , newSize, AnchorPosition.MIDDLECENTER); 
                    }
          catch(e){alert(doc.activeLayer + " Layer not resized ");}
          app.preferences.rulerUnits = startRulerUnits;
}

rome1304 wrote:

 

Hey Paul thanks for  your answer, however this is  more of a problem because I have to make the input manualy and one layer at a time. I'm looking  for something like a fit image to 6" but aplied to selected layer"

Fit Image is a plug-in script thate resizes documents. You may be able to modify the fit image script and replace the resizeimage with a process like Paul provided here for selected layers. The Fit Image dialog has both width and height fields and support actions. So you can record the use of your plug-in script in an action which will record the width and height into the action step and the plug-in will bypass displaying its dialog when the action is played. Remember Layers can be larger then and smaller then canvas size as well as canvas size.  So the layer transform may be up or down in size.  You will need to use layer bounds not document width and height like Fit image does for its calculations.  If you make the script a plug-in get a new uuid for it. You may also may want to have an option like fit image not to scale up if a layer fits as is.