Google recently made some changes to the layout of their results pages. Titles now appear at a bigger font and without underlines. Beyond just changing the look of the SERPs this has also affected out Google handles titles tags slightly. According to research posted on Moz, the safest number of characters to keep your titles from being cut off is now 55. This number is somewhat arbitrary as the actual length is determined by pixels, not characters. The bolding which shows the search query in the titles also eats into the allowed pixel width. 55 characters seems to provide the best balance between safe title length and enough space to write a good title. The linked to article also includes a tool to see what any given title might look like in search results.
This article discusses how Google is now using pixels to determine how much of the title tag to show in search results rather than number of characters. It’s includes the results from their tests as well as a tool for measuring pixel length of an entered title. Based on their results the maximum length of a title tag seems to be about 500px.