Thumbnail for excerpt are much too big

This is probably caused by the theme you are using. If is is displaying the Featured Image with out any adjustment, it will be very large. If you can edit the theme, you can add a width to the CSS for the Featured Image.

You can also add a tag in your CSS to control the size. Each icon is in a Div tag:

<div id=”appStore-icon-container”>

So for example:

#appStore-icon-container img {
max-width: 128px;
max-height: 128px;
}

Thank you. I modified my custom theme with your suggestion and it works.