Please, please, please… I'm not starting a debate on the pros and cons of scrollbars! I'm just showing how to add them if you need them…
The following is for SharePoint 2010 and Office 365 / SharePoint Online.
Change How the Page Scrollbars Work
Below is the before and after of Kyle's changes. While the cosmetic changes are minor, Kyle lists additional benefits in his article.
Add Scrollbars to Quick Launch
Note: If you just want to tighten up the Quick Launch area then see this article: http://techtrainingnotes.blogspot.com/2011/10/sharepoint-2010-365-tighten-up-quick.html
When you scroll down in a SharePoint page to find what's at the end of the Quick Launch area you also scroll the top navigation area out of site. This area has the site title, top link bar, search and the Tags and Notes buttons. To be able to browse the Quick Launch area without scrolling the entire page add this CSS to your master page:
If you want to scroll the entire left navigation area then use this CSS:
Setting the height of this area gives you a little bonus of adding text or links just below the navigation area. Search for "MSO_ContentTable" in your master page and place your content between the two "</div>" tags above this line. (This example is for the standard v4.master page. Yours may be different.)
Add Scrollbars to the Content Area of the Page
Add Scrollbars to a Web Part
This one is easy as it can be done from the web part's properties panel. Just click the web part's dropdown menu, click Edit, expand the Appearance section and enter the height.
Here's the result:
If you really want to force a web part or all web parts to display at a fixed size you could set the .ms-wpContentDivSpace class. Most likely you will want to do this at a single page level instead of in the master page.
If you want to change the height of a single web part then view the source of the page and search for "#MSOZoneCell_WebPartWPQ" until you find your web part. Copy the name that you find there, including the digit at the end and add it to your style selector.