Using hook_form_alter on an existing form element
Not sure which checkout form you try to alter, however, I've used mytheme_form_commerce_checkout_form_checkout_alter(&amp;$form, &amp;$form_state, $form_id) and mytheme_form_commerce_checkout_form_review_alter(&amp;$form, &amp;$form_state, $form_id) successfully.

And if I understand it correctly, the element 'edit-account-login-mail' already exists in your form. Therefore, I think you might want to add the size property instead of overwriting the form element, for instance:

$form['edit-account-login-mail']['#size'] = 40;