How do I make the form respond to the space it's in

You theme isn’t providing much support but this will get you closer to what you want:

.salesforce_w2l_lead input, .salesforce_w2l_lead textarea,  .salesforce_w2l_lead select{
width: 100%;
}

.salesforce_w2l_lead .sf_field_captcha img{
display: block;
}

.salesforce_w2l_lead .sf_field{
padding: 5px 0;
}