Did you search the board first? ::)
I'm a newbie too, and was having the exact same question. A quick search for "allvideos different size" brought your question first - and then some answers.
for example:
forum.joomlaworks.net/index.php?topic=8067.0
haven't checked this solution yet, but looks promising.
so that's really my advice to you as a newbie: not the above code itself, but the ability to perform good searches before you post