Showing the attachment for a page?

Use,

wp_get_attachment_image_src( $attachment_id, $size);

Specify the size parameter as follows;

$size (string/array) (optional)

Size of the image shown for an image attachment: either a string keyword (thumbnail, medium, large or full)
or a 2-item array representing width and height in pixels, e.g.
array(32,32). As of Version 2.5, this parameter does not affect the
size of media icons, which are always shown at their original size.

Excerpted from the WordPress Codex on wp_get_attachment_image_src

Your final code might look like,

echo '<img src="'.wp_get_attachment_image_src($attachment->ID, 'small').'" />';

Or

echo '<img src="'.wp_get_attachment_image_src($attachment->ID, array(100, 100).'" />';

If you want to specify via array.

Use,

• http://codex.wordpress.org/Function_Reference/add_image_size

…to specify custom image sizes.