Look at the source of the page that you're placing the in_place_editor_field onto.
There's probably a class="in_place_select_field" or something similar that you can use in your CSS to style the field however you want.
The place I copied from in my own application is from an Ajax.InPlaceCollectionEditor so you probably have something like class="in_place_text_field"
-Rob
Rob Biedenharn http://agileconsultingllc.com Rob@AgileConsultingLLC.com