FINALLY it's FIXED!!! Since November 2013 this print scaling problem has existed, starting in the 1st release of Mavericks up through Yosemite 10.10.2.
The release of Yosemite 10.10.3 on 4/8/15 has FINALLY fixed the problem.
I thought the latest HP Printer Software Update (Version 3.0 on 4/10/15) had something to do with it, when it appeared in my Mac App Store update record. But it appears to be the same update listed on the Apple Support site as HP Printer Drivers v3.0 for OS X from 9/30/14. The HP driver on my iMac for my DesignJet 800 shows a Driver Version 30, while the other machines in our office show completely different driver versions.
All of our machines with Mavericks or newer exhibited the problem, but I just updated one of our other iMacs from 10.9.5 to 10.10.3 and that FIXED the problem (with no changes to the existing installed HP drivers).
Update to OS X 10.10.3 and that should let you print to custom page sizes again (at least on HP printers).
BTW: It FIXED the problem on our OfficeJet Pro K8600 (13x19) printer too. I think its safe to say, it was always an OS X only problem.