BIMI assertion records can actually exceed 255 characters, because (quoting draft-brand-indicators-for-message-identification-05, § 4):

Per DNS [RFC1035], a TXT record can comprise several "character- string" objects. BIMI TXT records with multiple strings must be treated in an identical manner to SPF Section 3.3 (https://tools.ietf.org/html/rfc7208#section-3.3).

This means that you need to split your BIMI assertion record’s text data in chunks each not exceeding 255 bytes, and receiving systems must concatenate each string together without adding any whitespace.