Currently, this isn't a built-in option, but you can add a little code snippet to check the size when the user clicks submit. Here's the code snippet:
https://formidablepro.com/knowledgebase/frm_validate_field_entry/#kb-limit-uploaded-file-size
You can add custom code like this to your theme functions.php or a new plugin. If you have additional questions about this, please post in our Help Desk for support:
https://formidablepro.com/help-topics/