So it is very easy....
Follow these steps:-
1) Go to Blogger and select the post/ create a new post which you want to give a scrollbar.
2) Click on it and make a style code for it(new post)
2) Click on edit and go to style code if you've made it.
3) add this code to the style:-
.scrollbox { height:100px width:400px overflow:auto; }
4) Change the height and width as per your requirements.
That's all.
Now you can view your post scrollable.