What printer are you using? I will relate an issue I recently encountered. I don't know if it applies in your situation or not. I use a Canon PIXMA Pro 100. I wanted to print an 8.5 x 25" print which requires a custom setting, and the custom setting was not available. At the time LrC was set to print borderless and the custom setting was not available. But when I unchecked borderless printing the custom setting became available and I was able to get the print size I needed. Just a suggestion, Hope this helps.
I am not familiar at all with that printer. However, I looked at the specs online. 11 x 14" is listed as one of the sizes that printer is capable of producing. Perhaps there is a user group for the printer that might be able to offer some assistance. Good luck in resolving the problem.