If you are familiar with HTML and CSS, you can use CSS to resize the widget. For example, create a new HTML file called example.html with this text:
<html>
<style>
#frame { width: 400px; height: 300px; border: none}
#frame {
-ms-zoom: 0.75;
-moz-transform: scale(0.75);
-moz-transform-origin: 0px 0;
-o-transform: scale(0.75);
-o-transform-origin: 0 0;
-webkit-transform: scale(0.75);
-webkit-transform-origin: 0 0;
}
</style>
<div id="wrap">
<iframe id="frame" src="https://thingspeak.com/channels/929488/widgets/126606"></iframe>
</div>
</html>
Now, open that in your browser and compare that to just opening the widget
