Need @returnCode For This EXEC (@sqlString)...

I want to pass a database parm to enlarge the maxsize for around 500 databases. Here is the primary script:
--====================================
declare @stringData varchar(200),
@stringLog varchar(200),
@databaseName varchar(25),
@returnCodeSize int
select @databaseName = 'ABC'
select @stringData = 'alter database ' + @databaseName +
' modify file (name = ABC_Data, maxsize = 1500MB, FILEGROWTH = 10%)'
print @stringData
exec (@stringData)

It works in above way.
But it did not work if
I use @returnCodeSize =exec(@stringData)

how could I get a return code from this exec. I can't use cmdshell since it is not an external operating command.

thanks
David