Marking 3U board size on a 6U board in Altium
I'm using Altium 17.1:


View/Board Insight - check Heads-up display: this displays the position of your cursor
Grids - Set global grid: you can set the snap grid to mm-s to makes positioning easier
Draw the lines on the layer you want (probably a mechanical layer)
You can also set the line coordinate in the line properties (double click, or right click, properties)
also you can use Place/Dimension/Linear to measure and display distances