I can answer my own question now.
In Printer Settings, in the Dimension panel, there is a selector for Printer Type. That should be set to Delta Printer with Cylindric Print Shape. But Repetier Host uses the Cartesian printer's size to calculate the default viewport for the 3D views.
So, to fix it, open that pane, temporarily select Classic Printer, fill in the Width, Depth, Front, and Left, then switch the Printer Type back to Delta. I found that leaving Print Area Height at 100mm gives a more useful viewport for the small objects I usually print, so I left that at 100.
Objects are correctly centered on the bed now, too.

- Temporarily select Classic Printer, then set these.

- Then switch back to Delta and hit OK.

- Now this is the default viewport.