The number and size of the extents (files) that make up the BI is determined by the database structure, which is specified with structure files and prostrct commands. The sum of all these extents makes up "the BI file" (aka "the before image area" aka "the primary recovery area").
BI blocks and clusters are unrelated to the number or dimension of the files. BI block size (in KB) is chosen with proutil dbname -C truncate bi -biblocksize n. BI cluster size (also in KB) is chosen with proutil dbname -C truncate bi -bi n. BI clusters are logical structures within the BI file.
The various screens where you see BI block size and cluster size are all telling you the same thing. Proutil describe is a little confusing as it gives you cluster size in units of 16 KB instead of KB, but if you do the math (1024 * 16 KB = 16 MB) it's the same size you see in promon screen 7 or R&D 1 9.
When you talk about the size of the BI file, there are two ways to look at it: the physical size (sum of the sizes of the extents) and the logical size (the number of active BI clusters * the BI cluster size). Promon 5 and R&D 1 9 both show you physical size, just in different units.
When you truncate the BI file (proutil dbname -C truncate bi) you are changing the file logically. And if the variable BI extent had grown, or was the only extent, the command also changes that file physically.
The command commits the changes it contains to the database extents so that the physical structure of the BI file can be reset back to what is dictated in the structure file. The next time you open the database after a truncate bi it allocates four BI clusters. If they fit within fixed-size extents then the files don't have to grow. If they don't, or if there is only a variable extent, the variable extent grows to a size large enough to fit four BI clusters. If you issue a proutil bigrow n command after truncate bi, it allocates n more clusters in addition to the original four.
In your case, if you have six extents of 2 GB each, they don't "grow". They are fixed-size; that's what the "f" in the structure file line items means. They stay at the allocated size (in KB) no matter what you do, including truncate bi. You can have at most one variable-size extent per storage area and it must be the last extent. If an extent isn't marked as "f" or "v" then it is variable.