Home Forums Hardware Mac Desktops & Notebooks Mac Pro monitor dongle?

I am using my old Mac Pro (late 2010, 5,1) for storage, connecting it via router/ethernet to my iMac. I view the Mac Pro via Screen Sharing. The screen on the iMac is 5K pixels wide, the largest size I can manage on the Mac Pro is 2K, so you can imagine the viewing area of the Screen Sharing window is pretty small.

Here's the problem: in order to keep the 1920px screen size for the Mac Pro, I have to have the computer physically connected to my unused 24" Dell monitor. If I pull the DVI display cable from the Mac Pro, the window size defaults to 1024px and the preference pane offers no other size options. I have to keep the DVI cable physically attached to the Dell monitor even though the monitor is switched off and not even plugged into anything.

Question: is there some sort of dongle or trick that will make the Mac Pro think it's got a large monitor attached?

Thanks.