How to add the condition to this script?

I'm not sure what you want to do with the conditional if statement, but here it is in you script, with an alert letting you know if the layer height is between 100 and 1000, and another alert if the response to the prompt is with the range of 100 to 1000:

function getLayerSizeToCopy(){
    var layer = activeDocument.activeLayer; //Grab the currently selected layer

    // Calculate length and width based on the rectangular bounds of the selected layer
    var length = layer.bounds[2]-layer.bounds[0]; //Grab the length
    var width = layer.bounds[3]-layer.bounds[1]; //Grab the width

    // Remove pixels from the length/width "200 px" => "200"
    length = length.toString().replace(' px', '');
    width = width.toString().replace(' px', '');
  
    if(width>100&&width<1000){//tells you if the layer size is within the range of 100 to 1000
        alert('height is in range')
        }
    else{alert('height is NOT in range')}

    var promptResult = prompt( "Layer Size: ", length + ", " + width );
    
    var promptArray = promptResult.replace(/\s/g, "").split(',');

    if(promptArray[1]>100&&promptArray[1]<1000){alert ('The number entered is in range')}//Tells you if the response to the prompt is within the range of 100 to 1000
    else{alert('The number entered is out of range.')}
    
    
}
getLayerSizeToCopy();