There is no 'dynamic' automatic dimension tracking function while we actually draw like in other CAD programs other than the absolute and relative coordinate information given in the Status bar as Husky has rightly pointed out.
However I can see how this may be of use so I have added a feature request for it here:http://www.qcad.org/bugtracker/index.ph ... ask_id=674
Another viewing option may be:
Selecting a certain entity (line, circle, arc etc.) will show up the properties of that entity in the 'Property Editor' (View > Property Editor, keycode - GP), these properties, colour, line weight, length, radius or even the coordinate positions can all be changed here in this property Editor which will in this case! dynamically change in our drawing.