i need to increase the character limit of the custom text box to 1.000
or maybe add one more custom text box.
your help is highly appreciated.
If you are talking about adding additional customizable fields to a product page then you need to code the product pages yourself.
See an example of customized product page here: https://support.wix.com/en/article/corvid-adding-a-product-configurator-to-a-wix-stores-site