Well, for DKIM 60 chars isn't much. The value I was trying to put was
"v=DKIM1; k=rsa; t=y; p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQC5d62Jd5ST5EySxgitfN7wstxmmvVA+E1oPrYm8EnXAphVd/9M5DJKGRh9jOOJ8W35iZaRlz6IMIacCtjzwblNHvh/XV0/bvDOSKiWTb+AAlMXD+Os6cdwp8Vxmc71p+ZfwidnAjfHOEOOFy6l25Yxi7RZU3txC+dnsYUaJDFv4wIDAQAB"
(that should be a single line but this input box breaks it).
So to me it's plausible that a length restriction is forbidding it,
while I think the DNS specs allows this just fine. I put a similar
record in for a different domain that's registered at zoneedit.com,
which uses a very different user interface.
There are some indications that DKIM is getting some support here
and there, so me thinks DNS editing tools should not stand in its way.