Customize data types in a generated model with hibernate
It's not unreasonable that a maximum length should be specified for a field that is backed by a VARCHAR. In some frameworks, for example Django (Python), a max_length parameter is mandatory to create a CharField, but optional for a TextField. This makes sense: how else can the framework know the size of the column. A VARCHAR column needs a max size.

Make the maximum size a conscious design decision when using a relational database as storage. If the data is text and you need to support arbitrary length, then in the database it should be stored as a TEXT field instead of a VARCHAR. For that, use @Column(columnDefinition="text") or @Type(type="text"), as @ferada said in a comment,
and as discussed in this related post.

You remarked in a comment that H2 used the type VARCHAR(2147483647).
That's an irrelevant implementation detail of how H2 supports TEXT type.
It's indeed a bit funny.