Automating the creation of a text layer for a user to manually add text.

I think you would be better off prompting the user for the text then add the text layer you could then choose an approbate font size once you know the number of characters the user want in the text.  You need to calculate the font size and position the layer for the text size.

var userText = prompt ("Enter Text you want to add for xxxxx ");

Text goes something like this it old code I did not understand how text worked when I wrote it. Some vars use here their sets are not are not shown like font name color etc. Some setting are commented our for I wanted to later position the layer via an action. Paragraphs can not be position by my action.

// save Document resolutiom and set it to 72 DPI restore resolutio before returing to user working units pixels //
// I player with ressolution using a var testres and a size factor this code is cut from a scrio I wrote not knowing how text worked //
text_layer = doc.artLayers.add(); // Add a Layer
text_layer.name = "Layer Name"; // Name Layer
text_layer.kind = LayerKind.TEXT; // Make Layer a Text Layer
text_layer.textItem.color = textColor; // set text layer color
/* Do not set TextType to Pargarph Text for StampEXIF so action can position text layer
text_layer.textItem.kind = TextType.PARAGRAPHTEXT; // Set text layers text type
 */
text_layer.textItem.font = fontName; // set text font
text_layer.blendMode = BlendMode.NORMAL // blend mode
text_layer.textItem.fauxBold = false; // Bold
text_layer.textItem.fauxItalic = false; // Italic
text_layer.textItem.underline = UnderlineType.UNDERLINEOFF; // Underlibn
text_layer.textItem.capitalization = TextCase.NORMAL; // Case
text_layer.textItem.antiAliasMethod = AntiAlias.SHARP; // antiAlias
/* Calulate font size to use for Text keep size same for landscape and portrait base on document size here I use 30 as max character length */
if (doc.width >= doc.height) {var fontSize = Math.round(doc.height / (30 * sizeFactor));}
else {var fontSize = Math.round(doc.width / (30 * sizeFactor));}
if (fontSize<10){fontSize=10}; // don't use Font size smaller then 10
text_layer.textItem.size = fontSize; // set text font Size
//text_layer.textItem.position = Array(textX, textY ); // set text layers position in and down textx and testy need a setting
text_layer.textItem.position = Array(textX, (textY + fontSize )); // set text layers position in and down for Stamp add in fontsize
textWidth = ((doc.width - textX) * 72/testres ); // Text width document width - offset
textHeight = ((doc.height - textY) * 72/testres ); // Text height document height - offset
/* Do not set Text Area  so action can position text layer
text_layer.textItem.width = textWidth; // set text area width
text_layer.textItem.height = textHeight; // set text area height
 */
try{text_layer.textItem.contents = userText;}
catch (er) {alert("Error Setting Contents..."); }