Hi Chris,
I took another look, this comes from the RFC 1771.
The fourth high-order bit (bit 3) of the Attribute Flags octet is the Extended Length bit. It defines whether the Attribute Length is one octet (if set to 0) or two octets (if set to 1). Extended Length may be used only if the length of the attribute value is greater than 255 octets.
The lower-order four bits of the Attribute Flags octet are unused. They must be zero (and must be ignored when received).
The funny thing is, they removed the part about 255 octets from a later RFC (RFC 4271):
The fourth high-order bit (bit 3) of the Attribute Flags octet is the Extended Length bit. It defines whether the Attribute Length is one octet (if set to 0) or two octets (if set to 1).
The lower-order four bits of the Attribute Flags octet are unused. They MUST be zero when sent and MUST be ignored when received.
255 octets/bytes is a lot it might be possible. Take a look at this screenshot:
The attribute length is 25 bytes in that screenshot. Maybe if you go grazy with AS path prepending you can go above 255 bytes 