Look for this code on or around line 94 (I use the Green Theme, your theme may be slightly different)
Open, qa-include / qa-widget-ask-box.php and look for the following code:
<TD CLASS="qa-form-tall-data" STYLE="padding:8px;" WIDTH="*">
<INPUT NAME="title" TYPE="text" CLASS="qa-form-tall-text" STYLE="width:95%;">
</TD>
Change 95% to your desired width.
Good Luck