(View at full screen for best results.)
Here's one way you can make your line lengths specific, and also draw with the Pen/Bezier tool, to make the nodes automatically connected. In the video, I used Guides. But you could set up a grid for snapping instead, if you prefer (Document Properties > Grids tab).
Also, you'll see I circled the Snap control bar on the right edge of the window. Snapping is a sophisticated feature, which takes some times to learn. But you can set up your Snap bar just like mine, to do this job.
(For future reference, it's best to only have the snap options that you need, for the particular task at hand. If you have all the options enable, Inkscape will try to snap to everything in sight, which is really very hard to control. So for newbies, I always suggest only having what you need enabled.)
There's one difference you'll see on your canvas. Normally, I don't keep the snap notification visible. (Too lazy to make the video again
) But they are visible by default. So when you click to place the node, you'll see a very small bit of text, that says.....I think it's going to say "node to guide intersection". Even if your mouse is not precisely on the intersection, Inkscape will snap the new node right to it. If you don't see the notification, or the notification doesn't say what you meant to snap to, then it's not on the intersection.
In Document Properties > Snap you can set what distance you want for snapping. Although I really think of it more as the strength of the snap, than the distance. If you set Always Snap, it makes a very strong snap from further away. If you set let's say for 10, you have to get closer, and it's harder to notice when the snap happens.
The other important thing to notice, when I switched to the Pen tool, notice that small white box on the beginning of the arch path. When you place the Pen tool in that box, it turns red to indicate you're in the proper position to continue the path. Click once in the box. Then move down to the first guides intersection, and then on until you're back to the end of the arch path.
Again, it's very important to click in the white box (which turns red when the mouse is in the proper position. Just click once.
Now you have 5 nodes creating the window shape, and they're all connected. I filled it with gray, to show that they are connected. Although if my mouse was not in the box when I started or finished, the window still would have been filled, mostly. However, it's really better to ensure closed paths, whenever possible (for reasons that you might not need yet).