Apply style attributes to the input and select fields..
As Balinor says...
modules/product_options/customer_options.tpl
Inputs...
Code:
<input style="width:100%;" id="po{$v.classid}" type="text" name="{$poname}" value="{$v.default|escape}" />
Selects...
Code:
<select style="width:100%;" id="po{$v.classid}" name="{$poname}"{if $disable} disabled="disabled"{/if}{if $nojs ne 'Y'} onchange="javascript: check_options();"{/if}>