alright cool.. but you should have tried more... you should always try harder to solve problems! :)

is very big. How can I change its size

you can simply adjust the size of the logo using html.. this is what i usually do..

you can use the width and height attributes in html. eg:

<img src="logo.jpg" width="250" height="200" />

and depending on the width and height of your logo.
in your case, i think it should look something like:

<<?php echo $tag; theme_logo(); ?>   <a href="<?php echo get_option('home'); ?>/"><img src="<?php bloginfo('template_url'); ?>/images/logo.jpg" alt="<?php bloginfo('description'); ?>" width="250" height="100" /></a></<?php echo $tag; ?>>

or do you mean the file size?(KBs/MBs)

and by the way, if my answers are solving your questions, please give me up votes. haha.. i need to increase my ranking in this site (i like being ranked high) :)

your code have a syntax error like it was reported.. :p

?>
<<?php echo $tag; <?php theme_logo(); ?>   <a href="<?php echo get_option('home'); ?>/"><img src="<?php bloginfo('template_url'); ?>/images/logo.jpg" alt="<?php bloginfo('description'); ?>" /></a></<?php echo $tag; ?>>

in line 2, you have :

<?php echo $tag; <?php theme_logo(); ?>

it is supposed to be something like:

<?php echo $tag; theme_logo(); ?>

so your final code should be something like:

?>
<<?php echo $tag; theme_logo(); ?>   <a href="<?php echo get_option('home'); ?>/"><img src="<?php bloginfo('template_url'); ?>/images/logo.jpg" alt="<?php bloginfo('description'); ?>" /></a></<?php echo $tag; ?>>

i think it is something like a wordpress theme or something right? anyways, the wrong thing is you have <?php and then <?php
try removing it and lets see if it will make a difference..