tinymce_sample.js

editor.windowManager.open( {
					title: 'A popup dialog',
					body: [
						{
							type: 'textbox',
							name: 'text1',
							label: 'enter your text',
							multiline : true,
							minHeight : 60,
							
						},
						{
							type:  'container',
							html: String( text1.length() ),
							label: 'This changes dynamically based on length of text entered above',
						}
					],

and then call that function inside of the html: block, or in the textbox with an onchange: or something?

I'm gonna put my actual plugin code with the new stuff up here, so that it becomes clearer.