Hello Zhang,
Thank you for reaching out. With regards to your concern, the limits for asset.image_asset.full_size.url in the API should match the limits in the web interface. There is no maximum limit documented for this field, however, a URL has a maximum limit of around 2000 characters before it will no longer function properly in some browsers. So, I would recommend to specify the URL within the practical limits. Let us know if you need any additional information.
Regards,
Nikisha Patel, Google Ads API Team