TAD Docs

The size of the architect determines this value. A precise value can be given using the Headsup Command Line But if you drag the mouse (in selection mode) to form a rectangle in any empty part of the screen; then the size of the architect can be changed to the rectangle you indicated

Why is the stepsize important?
The stepsize has several uses. For one; it indicates the distance the architect would move if you press the arrow-keys on your keyboard once.

The stepsize also indicates the square which will be used as a tolerance when snapping intersection points…. so if you find that there are lots of intersecting lines and TAD can get confused on which intersection point to snap to; a smaller stepsize will give a finer granularity.

The architect can also act as a place to end the dragging of objects – if you drag the current object and/or set of objects and then leave the dragging action over the architect; then TAD magically will finish the drag precisely on the top-left corner of the square shape of the architect