Hi @cliqueradar,
Currently, there is no such a feature to limit the number of characters allowed for the additional notes field.
This field works in the same way as the default WooCommerce checkout.
If you need this field to allow customers to add a gift message, Fluid Checkout PRO does have a specific feature for that with which is possible to limit the number of characters.
Otherwise, I added a task to the backlog to analyze and possibly add the limit also for the additional notes field. However, this will likely take quite a while until I’m able to work on this new feature.
Best,
Diego