Hello:
How can add and preview .swf files.
Thanks in advance

You mean like images through [img] BBcode? Yes, if you'll modify bb_codes.php file and add one more bbcode for swf. Current version of miniBB has only image insertion option.

The following codes SHOULD work (I've just tested them on my server, so if you are experiencing an error, try to find it yourself):

encodeBB():

$pattern[] = "/\[flash\](.+?)\[\/flash\]
/is";
$replacement[] = '<OBJECT classid=clsid:D27CDB6E-AE6D-11cf-96B8-444553540000><PARAM NAME="movie" VALUE="\\1"></OBJECT>';

decodeBB():

$pattern[] = "/<OBJECT classid=clsid:D27CDB6E-AE6D-11cf-96B8-444553540000><PARAM NAME=\"movie\" VALUE=\"(.+?)\"><\/OBJECT>/s";
$replacement[] = '[flash]\\1[/flash]';

Thanks...

How can we change size of flash make it bigger or smaller?

It depends on how your flash movie is set up via HTML params... probably there must be some additional code to pass in <param> vars or URL itself.