I use a block just like this one almost every day:
http://www.autobodytoolmart.com/9-12-sa ... 12683.aspx

As R/T said, the 9" blocks work excellent and you'll find yourself reaching for them all the time. I also use the 3M stickit 18" Straight board a lot for leveling after skim coating or leveling & shaping filler.

What R/T said is spot on, you'll always want to use the longest block possible to get razor straight panels. A mistake a lot of people make is using small soft blocks on large panels, because they "seem" to cut better and faster. However, they don't level and can put waves and ripples in the work.