I remember seeing something like this somewhere and i noticed that it wasnt well implemented so i made my own. NONE OF THE ORIGINAL CODES BY THE AUTHOER WHO'S NAME I CANT REMEMBER HAS BEEN USED!

Step1:
Goto AdminCP => Custom BBCodes => Manage BBCodes => Add new BBCODE

Step 2:

Title:
iFrame BBCODES

tag:
if

replacement:
You can adjust the size of the window by editing these two lines:
Example:
[if]http://www.google.com[/if]


Discription:
iFrame BBCODE

Use {option}
No

Screenshots:




Please click install if you used this ;)