How to make a fluid box transition

I want when you click in the radio button for change the content, and when the box change the height, have a fluid transition. I have tried with transition property but don't work =/

MY CODE

.main-form-show {
     background: #fff;
     padding: 20px;
     max-width: 500px;
     margin: auto;
     border-radius: 10px;
     margin-top: 250px;
     transition: 1s;
    }

Can anyone help me?

Thanks!