Not UE4 per say, but there are a few ways to accomplish this.
EDID information for your display contains meta data on your screen’s physical sizing. You could use NVIDIA API’s to retrieve EDID. NVIDIA Windows Management Instrumentation SDK | NVIDIA Developer
Alternatively you could retrieve the resolution and DPI by other means, and divide to get physical size.
For testing, DPI can be retrieved on windows with following in cmd:
wmic path Win32_DesktopMonitor get /format:list
Example
A 32 inch 2k screen (30.5 viewable)
2560 x 1440 Resolution
96 DPI
2560 / 96 = 26.66 inches wide
1440 / 96 = 15 inches tall.
Confirming the diagonal size: 15^2 + 26.66^2 = 30.59 inches