You can work on some moves just standing in one spot, so you only need enough space to stand in a Bow stance in a pinch, or the opportunity presents itself to do a little mini training session. (There are more opportunities if you don’t mind looking strange in public).

Having said that, most Xinyi Liuhe Quan practice happens in straight lines, stepping out in one direction, and returning along the same line. This line I'll refer to as "The Walkway" here. 

The recommended length for your Walkway is between 6 to 10 big strides. It is usually between two fixed objects, preferably trees or poles. 

The length means you will get sufficient practice of body turning moves (ie moves where the body turns 180°), which are very important. The trees or poles provide a guide and a target to focus on (and later, to hit).

As for width, you can practice a lot, even if the path you have is quite narrow. But some moves have diagonal steps, so a good width for your Walkway is about 1-1.5 big strides wide.

The ground should be fairly flat and even.

Apart from the size of the training area, there are also some nice to haves.

Outside is preferable. It’s just usually more pleasant. Dirt or grass (which will become a dirt track in a week or so of daily practice) is a great surface to practice on. 

Somewhat secluded or private would be ideal. You are going to be moving your body in strange ways and making sudden, loud noises. That might attract some attention which will distract you.