That is a long requested feature. As far as i know Red51 plans to implement such in a later coming building-update.
Until then you could use the "size" command (command console - then size x x x) and place some kind of "references" somewhere.
To measure the distance i usually build with mostly the same sized beams and build a straight line from A to B in a different textured beam. Then every i.e. 5 beams i place an extra one to mark that 5-beam-mark.
That is a bit of effort but works for now.