Brand new to Draftsight, and 'playing around' with trying basic shapes before i submit the finalised drawing for laser cutting.

My problem is i cannot fathom out length of straight line. from A to B i have drawn  is is an inch,a foot ,or a mile. All i see is an undimensioned object.

So how do i tell the programme that i want to draw a semi circle of 3.25 inches radius. I think once i know how to tell i that all the other commands should fall into place. I am still of the old pen and compass era,but would really like to send it off to the firm so they can just plug it into the machine and go, knowing that i will get the right sized  piece,and not something that is the wrong size without them transferring everything from my pen and ink drawing to their cutting programme.

Thanks

Ron