By default, the size of the preview thumbnail in Symphony is set to a maximum height of 256 pixels (Figure 1):
Figure 1 Default Image Size
Note: The image could be smaller if the original picture is less than 256 pixels
If a customer wants to make the preview thumbnail larger, use the min-height variable to set a specific size as desired. To do this, we need to specify in the style attribute the min-height for the img element (highlighted in red):
<img style="min-height:350px" src="https://symphony.com/assets/brand/thumb_symphony_shards.jpg"/>
The code above will then render the larger image displayed below in Figure 2:
Figure 2 Larger Image Size
Please refer to this article for further information.