But that area will always be a rectangle (what you can see on your screen), so it might make you download many tiles that you don't need, if you already know which route you will take. So Maps Pro allows you to download the map image tiles along a path.
First of all, you will need the path in a format Maps Pro can load, such as KML or GPX. Easiest way is to head to http://geojson.io/ and draw the line you want, then save > KML. It will get you a KML file on your hard drive. Right-click it > Open with > Maps Pro.
Another option is to export a route calculated with Maps Pro to GPX, like in this screenshot:
Menu > Offline > Download this map >
You will now get an option to only download around a path, with a certain distance from the road: