1 block side is 0.5m you can convert that to inches and calculate from there.
setp is positioning precision not length, for instance setp 1 means that each time you move a beam/plank/etc. with the arrow keys/PgUp/PgDn after you have locked it into position with RShift it will move by 1mm
if you want to get lengths you should use size, I think size 1 1 1 on a beam makes it block size but you should experiment with that.