USCIS does not provide guidelines to geographical limits of a regional center. However, economic reports, job creation, increase of regional production and business activity reports must be filed to prove the need for a regional center within its geographical area.
There are no hard and fast rules on how much area a regional center can cover in terms of land and geographical locations. The main issues you should focus on are how you can meet the requirements for forming a regional center, business plan, etc. I would advise working with an EB-5 attorney on your plan to get off ground a successful regional center.
The EB-5 regulations do not limit the geographic size of a regional center, but there may be other factors, demographic, economic, or social that might make it more difficult to obtain a designation for a larger area. Usually, a regional center is comprised of one county or a collection of counties that are economically interdependent. There have even been a couple of regional centers designated as state-wide, but this was years ago. Now, each regional center application must be specific as to industry scope and geographic area. Each different parameter may require a different business plan and economic analysis report. Therefore, costs and time can be limiting factors for a large regional center proposal.
The Regional Center geography area will be dependent on the EB-5 project business activities and respective required employment creation in those areas affected by the business activities. Some regional centers cover the whole state, others cover more than one state, while others only cover a few counties.
USCIS does not provide strict guidance on the size of the Regional Center, but you will need to support whatever geographic area you choose with job creation, expenditures, and business activity. Consult with EB 5 counsel who can cooperate with an EB 5 economist.