Attack of the 50-foot Save Sheet
According to Gotow, what happened is that Apple changed the file dialogs so that the title bar is now considered to be part of the window—and changed the math everywhere except in save sheets.
Hopefully Apple will fix this in a future Yosemite update. In the meantime, if you use an app that saves files via the sheet style, you might want to remind yourself to shrink its height a bit every so often.
If a sheet does get so large that it goes off the bottom of the screen, you can delete the NSNavPanelExpandedSizeForOpenMode or NSNavPanelExpandedSizeForSaveMode default.
Update (2014-11-26): Daniel Ericsson suggests holding down the Shift key while resizing the side of the sheet.
Update (2015-04-09): This is fixed in Mac OS X 10.10.3.
