Database Files With Differing Sizes

This warning states that a database data file is a different size to the other data files in the same filegroup. To ensure proportional fill between the data files and, therefore, optimum performance, all data files in the same filegroup should ideally be of equal size with similar free space.

Suggested Action

Set all data files that are within the same filegroup to have the same size and auto-growth settings.

How to Increase\Decrease the Size of SQL Server Data Files

USE [master]
GO
ALTER DATABASE [WhippetWorks]
MODIFY FILE
(
    NAME = N'WhippetWorks',
    SIZE = 128MB
)
GO
DBCC SHRINKFILE(N'WhippetWorks', 128)
GO

Further Reading

​Additional Data Files and Proportional Fill | SQL Server Central

DBCC SHRINKFILE (Transact-SQL) | Microsoft Docs