Is there any limit of the characters in DNS TXT record?

Yes. Since the value of the TXT record has a limit of 1024 characters, it can not be registered with a character string exceeding the limit.

# Sample
<"v=DKIM1; k=rsa; p=MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEA2JDwscTV7jU5Xd86DY8Ts0aMgp6KchyxBn5rM/6qp8/ZPeMSeDjFCGRt+sqRqUGkQ3xjn4n4x0YMKgDXQpNNz/btOG0YBPF96pPFL1G+IbOst84tMkrFw3iRtdNeReolj2e87bjCHyVy3fDHOrkOuvSSeymAA16NvnMPgCsAp1oBjCTGmOQ8NvIMqS5lr2raha41KGwSdnEigbzIL8p4SoVjn3mSIBs8zJ/PL70rA9fCDckj71qwYB53lnEpSwpZImPVTnbudBAeooiiHTySHzIorkk4hCZmypcT8eVhnwiK0tGW3n4XGtwRuyIB9d7BJ2cGHSp/sWNQrqba6CeHowIDAQAB">

# For ECL 2.0 DNS, You don't need to split the values not to exceed 256 characters like this way:
<"v=DKIM1; k=rsa; "
"p=MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEA2JDwscTV7jU5Xd86DY8Ts0aMgp6KchyxBn5rM/6qp8/ZPeMSeDjFCGRt+sqRqUGkQ3xjn4n4x0YMKgDXQpNNz/btOG0YBPF96pPFL1G+IbOst84tMkrFw3iRtdNeReolj2e87bjCHyVy3fDHOrkOuvSSeymAA16NvnMPgCsAp1oBjCTGmOQ8NvIMqS5lr2raha41KGwSdnEigb"
"zIL8p4SoVjn3mSIBs8zJ/PL70rA9fCDckj71qwYB53lnEpSwpZImPVTnbudBAeooiiHTySHzIorkk4hCZmypcT8eVhnwiK0tGW3n4XGtwRuyIB9d7BJ2cGHSp/sWNQrqba6CeHowIDAQAB">