This is very simple. First open your blog and Go to Template and Click on Edit html button as shown in this image. Click on proceed on next window.
Now click on “Expand widget templates” check box and search for Read How to search Specific code in Blogger template. Now paste the following code Before / Above tag
You can change those blue color values as you like. First one is the length of the text summery if you don’t have an image on your post. second number is the text length if you have an image on your post to use as a thumbnail on the home page with your summery. Third and fourth numbers are the width and height if the thumbnail you are going to show in your home page with your text summery. IF you have larger or smaller images than this size you given the code those images will be changed to this size when creating thumbnails.