there are many auto read more post summary scripts around the web based on javascript this method is based on pure xml without any javascript used.
this method will change every post in your index pages to a snippet of small thumbnail , summary and a read more link easily without pain just do the following steps:
1- Go to Blogger dashboard choose from sidebar
Template > Edit Html > tick
click CTRL + F to open search box and find <data:post.body/> then replace it with the following code
<b:if cond='data:blog.pageType == "item"'>
<data:post.body/>
<b:else/>
<b:if cond='data:blog.pageType == "static_page"'>
<data:post.body/>
<b:else/>
<b:if cond='data:post.thumbnailUrl'>
<div style='clear:both;'/>
<span style='float: left; border: 1px solid #eeee; width: auto; height: auto; margin: 5px;'>
<img expr:src='data:post.thumbnailUrl'/>
</span>
<b:else/>
<span style='float: left; border: 1px solid #eeee; width: auto; height: auto; margin: 5px;'>
<img src='https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgY-wEcnUmKHGaG7FYI2q2UeWYPE1ax6CkFqeX9ezvglaeLfN2gb911Tfg13_L3r6kqUawMNs06jOxgtjMVpo5iF6USUDyLFMxoHlGJe6fM70I38cgwDbVx7XD99W2EDkTDif8IWrOaCwU/s72-c/nophoto.png'/>
</span>
</b:if><div style=''><data:post.snippet/></div>
<div style='clear:both;'/>
</b:if>
</b:if>
You can change the width and the height of thumbnail by changing the values in red "auto" by any cusom values e.g. 100px
Note that this method is simple and doesn`t need to much code yet you can`t customize the no of characters in summary since it`s limited to 143 letters
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