I've read a lot about it is preferable to have a regular square of rectangular lot. I have not found very much source on larger plots of land though. My question is if my house is on a large irregular(not of any shape) lot (~20 acres) and much of it is not used as you can imagine. Does this affect the energy of my home negatively? Is there a way I can demarcate it so that it seems like the house is one a regular size lot?

These are some considerations:

1. Often a piece of land tend to follow the contours or a site.

1.1. For example often if there is a stream etc... or an already existing neighbour, one do get irregular shape of land.

1.2. For example, in an urban setting especially New York, one tend to see artificial square / rectangular grid of land.

2. The key success factor of what you described has more to do with Location, location & location.

3. In fact, Feng Shui in the past is about Feng Shui of: a. Landscape, b. The Site and c. The building or home.

3.1. Thus your consideration is the middle = b. The Site.

3.2. In Feng Shui, The Site need not be exactly rectangular or squarish.

4. When one zooms into the home, often a rectangular shaped is common place.

Hope this helps!