Headers can contain long things yes. Cookies can pile up for example, all in one header. But that's not the case on the skins.lfs.net domain.
Trying a request on lfsmanual.net which is still behind CF:
< HTTP/1.1 200 OK
< Date: Wed, 28 Oct 2020 15:33:34 GMT
< Content-Type: text/html; charset=UTF-8
< Transfer-Encoding: chunked
< Connection: keep-alive
< Set-Cookie: __cfduid=dc9140hced7c7ae3f57372d87a9ada2251703779214; expires=Fri, 27-Nov-20 15:33:34 GMT; path=/; domain=.lfsmanual.net; HttpOnly; SameSite=Lax
< X-Content-Type-Options: nosniff
< Content-language: en
< X-UA-Compatible: IE=Edge
< Vary: Accept-Encoding, Cookie
< Expires: Thu, 01 Jan 1970 00:00:00 GMT
< Cache-Control: private, must-revalidate, max-age=0
< Last-Modified: Wed, 01 Apr 2020 10:22:26 GMT
< Vary: Accept-Encoding
< CF-Cache-Status: DYNAMIC
< cf-request-id: 061170e24000002cef2eb6c000000001
< Expect-CT: max-age=604800, report-uri="https://report-uri.cloudflare.com/cdn-cgi/beacon/expect-ct"
< Report-To: {"endpoints":[{"url":"https:\/\/a.nel.cloudflare.com\/report?s=BSoxSfy4R6nmTsRwhaOuQghY6tfvpNs3DPGY45n39SooOIrjgIBiUqRvFuQ5T%2BBN7lkq%2BH%2FfVTvqmSqhdfMdiFfN97Tu7lorKRemqVejVxAK"}],"group":"cf-nel","max_age":604800}
< NEL: {"report_to":"cf-nel","max_age":604800}
< Server: cloudflare
< CF-RAY: 5e95b749ffbc2cef-LHR
There's a somewhat long header of about 200 bytes - but that's the longest there.
In the http spec there are no limits defined. But servers have something like 8k for max length of a single header. Browsers seem to go for 10k-256k (!) I guess just to be safe?
Anyway, 768 bytes sounds reasonable in this case. But you cannot guarantee there will never be headers longer than that.