something like this...

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)