Resize Text Size based on Canvas Size

Try this script

var old_units = app.preferences.rulerUnits;
app.preferences.rulerUnits = Units.PIXELS;
var text_size = Number(activeDocument.activeLayer.bounds[2].value) - Number(activeDocument.activeLayer.bounds[0].value);
if (Number(activeDocument.activeLayer.bounds[0].value) < 0 || Number(activeDocument.activeLayer.bounds[2].value) > Number(activeDocument.width.value))
    activeDocument.activeLayer.translate(Number(activeDocument.width.value)/2 - (Number(activeDocument.activeLayer.bounds[2].value) + Number(activeDocument.activeLayer.bounds[0].value))/2)
if (text_size > Number(activeDocument.width.value) || Number(activeDocument.activeLayer.bounds[0].value) < 0 || Number(activeDocument.activeLayer.bounds[2].value) > Number(activeDocument.width.value))
    {
    var s = activeDocument.activeLayer.textItem.contents;
    var size = activeDocument.activeLayer.textItem.size.value;
    var prev_is_digit = false;
    var pos;
    var len;
    
    var x = new Array();
    for (var i = 0; i < s.length; i++)
        {
        var c = s.charAt(i);
    
        if (c >= "0" && c <= "9") 
            {
            if (!prev_is_digit)
                {
                pos = i;
                len = 1;
                prev_is_digit = true;
                }
            else 
                {           
                ++len;
                }
            }
        else
            {
            if (prev_is_digit) 
                {
                x.push([pos, len]);
                prev_is_digit = false;
                }
    
            }        
        }
    
    if (prev_is_digit) 
        {
        x.push([pos, len]);
        prev_is_digit = false;
        }
    for (var i = 0; i < x.length; i++)
        set_text_style(x[0], x[1]-1, size, -50);  
    text_size = Number(activeDocument.activeLayer.bounds[2].value) - Number(activeDocument.activeLayer.bounds[0].value);
    if (text_size > Number(activeDocument.width.value) || Number(activeDocument.activeLayer.bounds[0].value) < 0 || Number(activeDocument.activeLayer.bounds[2].value) > Number(activeDocument.width.value))
        {            
        activeDocument.activeLayer.translate(-activeDocument.activeLayer.bounds[0].value)
        activeDocument.activeLayer.resize(Number(activeDocument.width.value)/text_size*100, undefined, AnchorPosition.MIDDLELEFT); 
        }
    }
app.preferences.rulerUnits = old_units;
  
function set_text_style(from, len, size, tracking)  
    {  
    try {  
        if (len <= 0) return;
        
        var d = new ActionDescriptor();  
        var r = new ActionReference();  
        r.putEnumerated(stringIDToTypeID("textLayer"), stringIDToTypeID("ordinal"), stringIDToTypeID("targetEnum"));  
        d.putReference(stringIDToTypeID("null"), r);  
        var d1 = new ActionDescriptor();  
  
        var list1 = new ActionList();  
        var d2 = new ActionDescriptor();  
        d2.putInteger(stringIDToTypeID("from"), from);  
        d2.putInteger(stringIDToTypeID("to"), from+len);  
  
        var d3 = new ActionDescriptor();  
        d3.putUnitDouble(stringIDToTypeID("size"), stringIDToTypeID("pointsUnit"), size);  
        d3.putInteger(stringIDToTypeID("tracking"), tracking);  
  
        d2.putObject(stringIDToTypeID("textStyle"), stringIDToTypeID("textStyle"), d3);  
  
        list1.putObject(stringIDToTypeID("textStyleRange"), d2);  
        d1.putList(stringIDToTypeID("textStyleRange"), list1);  
  
        d.putObject(stringIDToTypeID("to"), stringIDToTypeID("textLayer"), d1);  
        executeAction(stringIDToTypeID("set"), d, DialogModes.NO);  
        }  
    catch (e) { throw(e); }   
    }  

If you know the total number of characters  the longest text line will have you most likely could come up with the font size that should be used so the length of the text layer will not exceed the width of the canvas. You may need to change the Documents resolution to 72DPI  without resampling calculate the font size to use add the text layer then restore the document ti its original dip resolution.    I believe I had to doe something like that to come up with a font size to use.  But I just hack at scripting and doe not know much about how Adobe text tool works.

Text is quite complex resolution, size, and number of text characters and which characters most fonts support proportional charter sizes all play a part.

You may be able to transform the text layer to fit within some area  by using the areas selection bounds and the text layer bounds to resize the text layer for the area.  However, I do not know if you do that, that the text will always be readable at the new transformed scaled size.

You can not put todays headline in a icon size image file and be able to read it.