When defining a computed column with an explicit type, the RDB$FIELDS.RDB$CHARACTER_LENGTH is populated. Doing the same without an explicit length leaves this column NULL.

Reproduction (using default character set UTF8):

CREATE TABLE TEST_COLUMN_METADATA (
    col_varchar_default_user VARCHAR(100) DEFAULT USER,
    col_varchar_default_literal VARCHAR(100) DEFAULT 'literal',
    col_varchar_generated_explicit VARCHAR(201) COMPUTED BY (col_varchar_default_user || ' ' || col_varchar_default_literal),
    col_varchar_generated_implicit COMPUTED BY (col_varchar_default_user || ' ' || col_varchar_default_literal)
)

col_varchar_generated_explicit will have

RDB$FIELDS.RDB$FIELD_LENGTH = 804
RDB$FIELDS.RDB$FIELD_TYPE = 37
RDB$CHARACTER_LENGTH = 201
RDB$CHARACTER_SET_ID = 4

col_varchar_generated_implicit however has:
RDB$FIELDS.RDB$FIELD_LENGTH = 804
RDB$FIELDS.RDB$FIELD_TYPE = 37
RDB$CHARACTER_LENGTH = NULL (0 in 2.5.8)
RDB$CHARACTER_SET_ID = 4

Based on the information available, Firebird should be able to populate RDB$CHARACTER_LENGTH with 201 for the implicit type as well.