DECLARE @sql VARCHAR(8000);

--Set your threshold here
DECLARE @big int=1000	--mb

;WITH dbs([dbid],size) AS
(	SELECT database_id, 
		SUM(size)/128
	FROM sys.master_files
	GROUP BY database_id )
SELECT @sql = COALESCE(@sql,'')+
		CASE
			WHEN size < @big  
				THEN 'BACKUP DATABASE '+d2.name+' TO DISK = ''C:\Backup\'+CONVERT(VARCHAR(20),GETDATE(),112)+ '.BAK'''+CHAR(13)
			ELSE 'PRINT ''REPLACE Me with your code to backup db to multiple files'''+CHAR(13)
		END
FROM dbs d1
JOIN sys.databases d2 ON d1.dbid=d2.database_id
WHERE d2.name NOT IN ('master','model','msdb','tempdb')

EXEC(@sql)

DECLARE @sql VARCHAR(8000);

--Set your threshold here
DECLARE @big int=50000	--mb

;WITH dbs([dbid],size) AS
(	SELECT database_id, 
		SUM(size)/128
	FROM sys.master_files
	GROUP BY database_id )
SELECT @sql = COALESCE(@sql,'')+
		CASE
			WHEN size < @big  
				THEN 'BACKUP DATABASE '+d2.name+' TO DISK = ''C:\Backup\'+CONVERT(VARCHAR(20),GETDATE(),112)+ '.bak'''+CHAR(13)+CHAR(13)
			ELSE 
				'BACKUP DATABASE '+d2.name+' '+CHAR(13)+
				'TO DISK = ''C:\Backup\'+CONVERT(VARCHAR(20),GETDATE(),112)+ '1.bak'','+CHAR(13)+
				'TO DISK = ''C:\Backup\'+CONVERT(VARCHAR(20),GETDATE(),112)+ '2.bak'','+CHAR(13)+ 
				'TO DISK = ''C:\Backup\'+CONVERT(VARCHAR(20),GETDATE(),112)+ '3.bak'','+CHAR(13)+
				'TO DISK = ''C:\Backup\'+CONVERT(VARCHAR(20),GETDATE(),112)+ '4.bak'','+CHAR(13)+ 
				'TO DISK = ''C:\Backup\'+CONVERT(VARCHAR(20),GETDATE(),112)+ '5.bak'''+CHAR(13)+CHAR(13)
		END
FROM dbs d1
JOIN sys.databases d2 ON d1.dbid=d2.database_id
WHERE d2.name NOT IN ('master','model','msdb','tempdb')

PRINT(@sql)