There is an easy to implement expandable post summary at Expandable Post Summary for New Blogger. The advantage of this is, it is easy to implement. The disadvantage - there will be a "Read More" link at the bottom of a short post even if you have no further content beyound what is in the post summary. I partly overcome this by putting an END OF POST at the end of a short post.
If you are still not satisfied with this, you can try to implement Hackoshpere's expandable post summary with peekaboo view. It is a bit harder to implement, but it has been done for this blog: Anything Chinese. You will see the "Read More" link at the bottom of the long posts in the main page, but I purposely wrote a short post Why the emphasis on things Chinese?. If you visit the homepage now, you will see that the other posts have "Read More" links at the bottom of the post but not for Why the emphasis on things Chinese?. "Read More" links will only be seen in long posts in the homepage, but not in individual post pages.
As more articles are published in this blog Anything Chinese, the short post without "Read More" link will disappear from the homepage, and what I am trying to show will disappear, so to make a permanent record of the homepage as it is now, I will post screenshots of the homepage below, with the "Read More" links circled in red to show it more clearly. You will see in the screenshots there is no "Read More" (remarks in green) link in the short post with the title "Why the emphasis on things Chinese" (click on screenshots to enlarge).