How to validate for file attachment size during list item save

I have a custom list form which implements out of the box attach file functionality to add attachments to the list item. Upon save, the attachments are saved to the item. But before saving the list item with attachments, I would like to validate for the following:

1. Limit only one attachment per list item.


2. Validate for attachment size which should not be more than 250KB.

Please let me know if the above can be implemented. Thank you.