Format column length in SSMS output
You need to use CONVERT(VARCHAR(xx), ColumnName) on all columns if you want the column to appear shorter in text output view.

Convert your query into something like:

SELECT [Server] = CONVERT(VARCHAR(30), SERVERPROPERTY('Servername')) 
   , DatabaseName = CONVERT(VARCHAR(30), '''' + bs.database_name + '''')
   , LastDatabaseBackupDate = CONVERT(VARCHAR(30), MAX(bs.backup_finish_date))
FROM msdb.dbo.backupmediafamily  bmf
    INNER JOIN msdb.dbo.backupset bs ON bmf.media_set_id = bs.media_set_id  
WHERE  bs.[type] = 'D' 
GROUP BY bs.database_name  
ORDER BY bs.database_name;


This will give output similar to:

Server                         DatabaseName                   LastDatabaseBackupDate
------------------------------ ------------------------------ ------------------------------
[ServerName]                   'A'                            Sep 25 2015 11:32AM
[ServerName]                   'B'                            Apr 21 2015 12:09PM
[ServerName]                   'C'                            Feb 24 2015  9:16PM
[ServerName]                   'D'                            Oct  8 2014 11:02AM
[ServerName]                   'E'                            May 14 2014  6:27PM

(5 row(s) affected)


If you want to be able to dynamically change the column widths without modifying the T-SQL code, you'll need to use Dynamic SQL:

DECLARE @ColumnWidth VARCHAR(4);
DECLARE @Cmd NVARCHAR(MAX);

SET @ColumnWidth = '24';
SET @Cmd = '
SELECT [Server] = CONVERT(VARCHAR(' + @ColumnWidth + '), SERVERPROPERTY(''Servername'')) 
   , DatabaseName = CONVERT(VARCHAR(' + @ColumnWidth + '), '''''''' + bs.database_name + '''''''')
   , LastDatabaseBackupDate = CONVERT(VARCHAR(' + @ColumnWidth + '), MAX(bs.backup_finish_date))
FROM msdb.dbo.backupmediafamily  bmf
    INNER JOIN msdb.dbo.backupset bs ON bmf.media_set_id = bs.media_set_id  
WHERE  bs.[type] = ''D'' 
GROUP BY bs.database_name  
ORDER BY bs.database_name;
';

EXEC (@cmd);


Here, I've set the width to 24 for all columns, and it comes out looking like:

Server                   DatabaseName             LastDatabaseBackupDate
------------------------ ------------------------ ------------------------
SERVERNAME               'A'                      Sep 25 2015 11:32AM
SERVERNAME               'A'                      Apr 21 2015 12:09PM
SERVERNAME               'A'                      Feb 24 2015  9:16PM
SERVERNAME               'A'                      Oct  8 2014 11:02AM
SERVERNAME               'A'                      May 14 2014  6:27PM

(5 row(s) affected)


If you really want to go crazy and have the columns automatically size themselves, you would do this:

DECLARE @ColumnWidthServer VARCHAR(4);
DECLARE @ColumnWidthDatabase VARCHAR(4);
DECLARE @ColumnWidthLastBackup VARCHAR(4);

DECLARE @Cmd NVARCHAR(MAX);

SELECT @ColumnWidthServer = 1 + LEN(CONVERT(VARCHAR(128), SERVERPROPERTY('Servername')))
   , @ColumnWidthDatabase = 1 + MAX(LEN('''' + bs.database_name + ''''))
   , @ColumnWidthLastBackup = 1 + MAX(LEN(CONVERT(VARCHAR(128), bs.backup_finish_date)))
FROM msdb.dbo.backupmediafamily  bmf
    INNER JOIN msdb.dbo.backupset bs ON bmf.media_set_id = bs.media_set_id  
WHERE  bs.[type] = 'D';

SET @Cmd = '
SELECT [Server] = CONVERT(VARCHAR(' + @ColumnWidthServer + '), SERVERPROPERTY(''Servername'')) 
   , DatabaseName = CONVERT(VARCHAR(' + @ColumnWidthDatabase + '), '''''''' + bs.database_name + '''''''')
   , LastDatabaseBackupDate = CONVERT(VARCHAR(' + @ColumnWidthLastBackup + '), MAX(bs.backup_finish_date))
FROM msdb.dbo.backupmediafamily  bmf
    INNER JOIN msdb.dbo.backupset bs ON bmf.media_set_id = bs.media_set_id  
WHERE  bs.[type] = ''D'' 
GROUP BY bs.database_name  
ORDER BY bs.database_name;
';

EXEC (@cmd);