How can we set the items per page to a view, programatically?

I had the same question.

I found useful info here:
http://pedroposada.com/blog/embed-views-2-nodes.html

$view = views_get_view($viewname);
$view->set_arguments( array( $arg ) );
$view->set_display($displayId);
$view->set_items_per_page($number);
$view->execute();
$count = count( $view->result );
print $view->preview($display_id, $args);

$args = array();
$view = views_get_view('viewName');
$view->set_display('default');
$view->display_handler->set_option('items_per_page', 1);
print $view->preview('default', $args);

nothing but the above code here worked for me

replace viewName by your view name and voila!

if you want to do the same thing without embedding the view, you'll need to create a module (trust me, there's no other way)
then use hook_views_pre_execute()

Thanks so much, alienzed. After a few hours of frustration, stumbling into your solution in #2 worked perfectly.

And where does one put this snippet of code? In a block, in a node, in a view, in a template? Sorry, I'm not strong on the coding side, but I would really like my users to set the number of results per page in a view. Can someone give me the "Dummies" version?

Well, I can tell you, don't put it into a block and apply it to the whole site. It will take your site down if it does not work. Believe me, I just learned the hard way. Thank goodness it was as simple as replacing the field content in the db to get it back.
If I figure out where to put it that makes logical sense and works, I'll be the first to share my results.

Best Regards,

Daboo

I found what I think is a much easier way of pulling this off if Views_3 is not an option yet.

using hook_form_alter apply the following to the appropriate $form:

//Add select number of pages filter.
$pager_options = array(20, 30, 50);
$form['views_page_filter'] = array(
'#type' => 'select',
'#options' => $pager_options,
'#default_value' => 1,
'#title' => t('Select items per page'),
);

$pages = $pager_options[$form_state['input']['views_page_filter']];
$form_state['view']->set_items_per_page($pages);

Though this will only work if you have an exposed filter for the view.

It works, thank you!