@Daniel did you figure this out? It’s on my list of things to check out as well.

if you look at the code that constructs the file upload meta box (\parts\fields\file.php), you find this at the top of it:

<?php
  $options = array_merge(
              array(
                'button' =>'Add Media'
                ,'modal_title' =>'Add Media'
                ,'basic' => false
                ,'preview_size' => 'thumbnail'
                ,'textarea_rows' => 5
                ,'save' => 'id'
              )
              ,isset($options) && is_array($options) ? $options : array()
            );
?>

So I’m guessing you can do this:

`piklist(‘field’, array(
‘type’ => ‘file’
,’field’ => ‘upload_basic’
,’scope’ => ‘post_meta’
,’label’ => __(‘Add File(s)’,’piklist’)
,’description’ => __(‘This is the basic upload field.’,’piklist’)
,’options’ => array(
‘preview_size’ => ‘thumbnail’ //change this i.e ‘medium’, ‘large’, ‘full’ etc
)
));`

Not tested, so let me know if it works