See a similar issue in Route 53 forum:
Unfortunately the 255 character limit per string on TXT records is not a Route53 limit but rather one imposed by the DNS protocol itself. However, each TXT record can have multiple strings, each 255 characters long. You will need to split your DKIM into multiple strings for your TXT record. You can do this via the console by entering each string encapsulated in quotes, one string per line.
Important note: Do not use “one string per line” as the instructions say — separate strings with a single space, eg.
"foo" "bar" not
"foo"\n"bar". Use DKIMValidator to validate the signature is being read correctly.