You can set up a template so that all of your new projects adopt the Project Settings (like grid size) of that template file. To set up a Template File, follow these steps:
1) Start with a New Project
2) Adjust your Project Settings and contents to your preference. Some examples:
a) File > Project Settings; here you can set your project scale, units, numerical accuracy, background color, etc
b) Grid Size
c) Materials
d) Layers
e) Lights
f) Objects (that you intend to use consistently)
(Anything set in this file will be in all of your new projects but will not affect existing files.)
3) When the file is arranged to your liking, save it somewhere where it won't accidently be deleted, such as "Documents/formZ folder."
4) Close the file
5) Go to Edit > Preferences, and choose the Project tab
6) Choose "Use Template Project" and click the Choose Template File button
7) Locate the file that you just saved and click OK to set it as the template, then OK again in the Preferences window.
Now your next new project should mirror the settings of your Template file.