the input.tnp-email comes with a width 100% (if the wp-content/plugins/newsletter/style.css is loading). The best way to change this is to
A: set a width in your css to the parent container .tnp-field.tnp-field-email or the form container .tnp.tnp-subscription
B: set a width directly to the input.tnp-email with the !important parameter.