This is a matter of personal preference.
You could have the first wood beam start right up against the wall.
You could leave some space between the first beam and the wall.
Or you could start with a half beam (one wood beam that you have cut lengthwise down the middle) up against the wall, placing the other half against the opposite wall. This method gives the appearance that half of each beam is embedded inside the wall (much like a true Timber-framed home would be framed).
The slideshow on the right shows these three different looks for your ceiling beams.
This too, is a matter of personal preference. The majority of customers install their wood ceiling beams with a spacing somewhere between 2 feet and 8 feet. Four-foot spacing is the most common, however.
The slideshow on the right may help give you a visual reference for when you install your own ceiling beams.