If you try to add a DKIM key that's generated by cPanel into OpenSRS while using their default nameservers, you might encounter the error, "Text length must be between 1 and 255 characters."
The cPanel generated DKIM key is a 2048 bit key. And the default SystemDNS nameservers, used by OpenSRS, supports only 1024 bit keys. If you use their default SystemDNS nameservers, you may receive the error as mentioned earlier when attempting to use a DKIM key that's longer than 1024 bits.
With your cPanel-generated DKIM key being larger than 1024 bits, you would need to set up and use your own nameservers to host your DKIM key. As OpenSRS allows you to specify nameservers, you can configure your settings in OpenSRS to use your nameservers instead of their default SystemDNS nameservers.
With this in mind, the following support article explains how to configure your own nameservers: How to configure nameservers in a cPanel & WHM server.