You have to use the frm_compact CSS class in the field's CSS Layout Classes field. This will give you the smaller single-line button. Then you can apply your custom CSS to .frm_compact_text > button.

In addition to @Victor Font's comment, you can also disable the dropzone after adding the frm_compact CSS class in the layout class fields.
 
Simply add this to your function.php child theme or code snippet plugin. 

add_filter( 'frm_load_dropzone', '__return_false' );