I found out the following way to solve the issue

__files/search-response-e.json

{{#assign 'page-size'}}20{{/assign~}}
{{#each request.query}}
  {{#if (eq @key 'size')}}
    {{#assign 'page-size}}
      {{this}}
    {{/assign~}}
  {{/if~}}
{{/each~}}

Iterating over the request.query content doesn't include the size property when it's not sent as a query parameter. So it is possible to iterate over all elements in request.query, check if any of the keys matches the parameter {{#if (eq @key 'size')}} and then assign the value replacing the default one if the parameter is present.

It is a solution but it's also very verbose and weird to understand what it's doing at first glance. I would appreciate it if anyone knows a better and cleaner way to solve this.