magento2 – Receiving checkbox status as Yes or No on a custom form to an email template

I am new to php and html and trying my hands on the below code where I have a custom form as shown in below SS and once customer submits the form I need to get the checkbox status as ‘Yes’ if the checkbox is checked and ‘No’ if it’s not checked to my email,

Custom form

Code for this form is under a .phtml file as below-

<p class="free-trial-wp"><input type="checkbox" placeholder="I 'd like a free trial "name="all_sizes" id="all_sizes" value="All" checked ><span></span>&nbsp; I'd like a <strong>free trial</strong></p>

code for email template is under .html file as per below-

      <!--@subject Quote/Demo Request @-->
  {{template config_path="design/email/header_template"}}
    <tr class="subscribe-email-intro"><td>Email: {{var email}}</td></tr>
    <tr class="subscribe-email-intro"><td>Organistaion: {{var organisation}}</td></tr>
    <tr class="subscribe-email-intro"><td>Contact Name: {{var contact}}</td></tr>
    <tr class="subscribe-email-intro"><td>Quote For: {{var productname}}</td></tr>  
    <tr class="subscribe-email-intro"><td>Contact Number: {{var telephone}}</td></tr>   
    <tr class="subscribe-email-intro"><td>Message: {{var comment}}</td></tr>
    <tr class="subscribe-email-intro"><td>Free trial: {{var all_sizes}}</td></tr>   
{{template config_path="design/email/footer_template"}}

Please let me know what changes do I have to make in both the files .phtml and .html to get the checkbox statuses as Yes or No. Thanks.