What are The main Determinants that should be Considered to Decide the Kinds of PLANTS that will be used in a large scale Landscape Project ?

Designing, installing and managing natural landscaping projects will vary in complexity and approach depending upon the nature of the site and the project goals.
There are different degrees of natural landscaping, ranging all the way from a small native flower patch, to a full-scale replication of a plant community covering many acres.
An otherwise traditionally landscaped site may be installed with a "natural" storm water lagoon with a fringe of native wetland plants and upland buffer to trap sediment.
Larger projects have the most at stake, financially and visually.
They require a detailed and technical planning process.
Some examples of larger scale projects include corporate campuses, university lands and large community parks.

Hello again.
Main determinants are:
1- The location and weather.

2- Amount of water supply, and the quality of irrigation.

3- Type of the soil: Clay or sandy or stone one...

4- Layers of the land and their depths, this's determined by a surveyor,so to make sure that length of your plants' roots do fit in the demanded layers.

5- Land dimensions: because plants need specific spaces between each other, depending on their types.

6- Land levels " Contour lines", if existed.
I recommend you read " FAB job to become a landscape company owner".

Physical conditions of site ( weather, soil,etc..)

Role or Function of plants in the project ( shading, aesthetics,aromatic,etc...)

Budget

Native plants and plants with low water and maintenance requirements  

You should try to develop your design to be  matched plants in colors, sizes and smells. Try to choose a gradient sizes and color , don't ignore the Semitic elements, similar positions and also the color's contrast, don't ignore the fruity trees, finally you should be aware and carefully about the required amount of water as your question mentioned by choosing plants have a low water requirements..

As already said Sherein Sorour