Magento 2 – How to Increase Attribute Length for Custom Attributes

It's Simple

$prod=Mage::getModel('catalog/product')->load($id);

$val=$prod->getData('attribute_code');

now assign the values to another attribute of that product

$prod->setAttreibuteCode($val);

$prod->save();

Don't forgot to save product.

The text attributes are stores in the table catalog_product_entity_text. Their values are stores in the value column that is of type text.
You can see here the max lengths allowed for all column types.

This is what's important to you:

BLOB, TEXT               L + 2 bytes, where L < 2^16   (64 Kilobytes)

You can also use long text for an attribute, but this is more complicated. You would have to create a table similar to catalog_product_entity_text but with the column value having the type LONGTEXT.

THe max storage for this is (from the same source)

LONGBLOB, LONGTEXT       L + 4 bytes, where L < 2^32

But you will also have to search through the code to see where you need to tell magento to consider your new table. I have no idea what this will take.

I suggest using the TEXT attribute. It should be enough storage. If you plan to keep more than 64k in one column you might need to rethink your architecture.