I spent some time looking into this:
Ever since Windows 8.1 we have per-display DPI settings - So you could have two monitors, and change the display scaling on the larger one that the presenters use, they could even have an entire wall with a projected image and custom DPI (projectors are cheap)
A second option would be, Per application DPI Scaling, so running two instances of the program, one for the stream output, and another for the presenters.
A third option, External computer magnification stands, used for the visual impaired, could be re-purposed to make the screen twice as big for the presenters.
Just thought it may be helpful to see some options, if you need funds for equipment just ask the community you have built, you are also welcome to use all my epic balance in such a venture as I believe tutorial videos (by Epic staff) are fundamental to Epics growth and the endorsement of UE4 as the leading engine.