You will need to edit "includes/view-functions.php" file for this. You can follow these steps :
1. For simple name field .
Go to line 691 and 695, you will see these codes
<input id="element_{$element->id}_1" name= "element_{$element->id}_1" class="element text" maxlength="255" size="8" value="{$element->populated_value['element_'.$element->id.'_1']['default_value']}" />
and
<input id="element_{$element->id}_2" name= "element_{$element->id}_2" class="element text" maxlength="255" size="14" value="{$element->populated_value['element_'.$element->id.'_2']['default_value']}" />
then change the "size" number in there.
2. For extended name field
Go to line 732, 736,740, and 744 for these codes :
<input id="element_{$element->id}_1" name="element_{$element->id}_1" class="element text" maxlength="255" size="2" value="{$element->populated_value['element_'.$element->id.'_1']['default_value']}" />
<input id="element_{$element->id}_2" name="element_{$element->id}_2" class="element text" maxlength="255" size="8" value="{$element->populated_value['element_'.$element->id.'_2']['default_value']}" />
<input id="element_{$element->id}_3" name="element_{$element->id}_3" class="element text" maxlength="255" size="14" value="{$element->populated_value['element_'.$element->id.'_3']['default_value']}" />
<input id="element_{$element->id}_4" name="element_{$element->id}_4" class="element text" maxlength="255" size="3" value="{$element->populated_value['element_'.$element->id.'_4']['default_value']}" />
and change the "size" value there.
MachForm Support