You can either add CSS along these lines...
Code:
div.ask-question a:link { font-size: 24px; ETC ETC }
Or you can turn the link into a button...
skin/smart_template/customer/main/product_details.tpl
Replace...
{include file="customer/buttons/button.tpl" button_title=$lng.lbl_ask_question_about_product style="
link" href="javascript
: return !popupOpen(xcart_web_dir + '/popup_ask.php?productid=`$product.productid`')"}
With...
{include file="customer/buttons/button.tpl" button_title=$lng.lbl_ask_question_about_product style="
button" href="javascript
: return !popupOpen(xcart_web_dir + '/popup_ask.php?productid=`$product.productid`')"}