Here’s a quick example. In this case I was only concerned with the longest dimension and I have no idea how long this garden is really supposed to be. I just scavenged an image out of a freely available PDF.
Assuming the garden to be 10 meters long, I used the Scaled Drawing feature to make a scaled 10m long line at 1:50. Then adjusted the size of the inserted image to match. You don’t need to add the dimension in there. I just did it for the example.
When you resize the image, use a corner handle and hold Shift to maintain the aspect ratio of the image. I suggested using a rectangle or two lines at right angles so you can confirm the aspect ratio of the scanned image is correct. My experience with scanned images is that the A.R. is often out by a little bit. Might not be a problem in this case but it’s good to know up front.