ok, now we're going. so, I think what i would do is use the make polygon tool to draw out the shape of the arrow - you should be able to get something pretty precise. You could also try using the curve tools , like the cv tools, but I think that might be more trouble than it's worth. so grab the create polygon tool - switch to a side or top view and then start clicking points to make the shape of the arrow and then enter when you're done.
For the actual dimensions - i would use the text tool and then simply convert the text to polygons once you're done.
you also might want to give everything a bit of depth by using the extrude tool otherwise it might disappear from certain angles when you rotate around the object.
if you have any problems just keep posting questions here and make sure you include a pic.
hope that helps.