Tips for New Bloggers: Add Scrollbars to Blog Widgets:
"Scrollbar in All Widgets
Now that we know what the code does, we can apply it to our template. If we have many widgets in our sidebar, we can specify a fixed height for all the widgets. Carefully planned, our layout can look very neat since all the widgets will have the same height.
Login and go to Template -> Edit HTML. Insert this piece of code. For easy reference, we have added it under the /* Sidebar Content */ :-
/* Sidebar Content */