Make booking form fit full page

do you have sample link to your site for us to see and analyze please? thanks

sorry but the bookings from the link above is already closed ?

seems that your theme css is conflicting with the form input css; you can try to add this at the end of your theme style.css or admin theme custom css section

.em-booking-form-details input {
  width: 20%;
}
.em-booking-form-details select {
  width: 25%;
}
.em-booking-form-details label {
  width: 30%;
}
.em-booking-form-details {
  width: 70%;
}