Impact on database while increasing database initial size
I want to increase its initial size to + 100 GB , while doing that in the properties -> filegroup->initial size, do I require a SQL Server restart or downtime after applying the changes?


No. Increasing initial size of the database should be transparent and does
not require a service restart.

If your database is highly active, suggest you to do this during less active time.

Below are my recommendations :


Enable auto growth to be in MB as opposed to a % increase.
Enable Instant File Initialization, this prevents the writing of zeroes prior to space allocation. Which means when auto growth or when you have to increase size of data files, it's extremely fast.


Refer to my answer for : Database defragmentation and autogrowth settings