I use dual monitors in addition to my laptop, Lenovo ThinkPad X1. One monitor is a Dell Ultrasharp UP2720Q 27" 16:9 4K and the other is a Dell 23.8" LED U2414H. My primary monitor is the 23.8" U2414H and the secondary monitor is the Ultrasharp UP2720Q 27". Once I moved LR into the secondary monitor, which is the one I want to use in the Develop module, all of the panes got huge because of the larger monitor size. But the display did not change when I dragged it back into the primary monitor. Now LR always seems to "think" I'm opening it in the larger monitor and all of the panes are too large and out of proportion, even when I unplug the larger monitor.
I have attached a screenshot of my main window in the primary monitor.
I made sure that the screen resolutions and scaling settings in Windows were the same for both monitors (100%).
I can't even resize the leftmost or rightmost panels even though I get a double-headed arrow when I hover the mouse over the borders between those panels and the center panel.
I tried rebooting the computer after disconnecting the secondary monitor and opening LR in the primary monitor, dragging LR to the other monitor then changing the primary & secondary monitor in Preferences.
I tried changing the high DPI settings (Properties>Compatibility>Change high DPI settings>Override high DPI scaling behavior) and restarting after applying, but the only thing that did was to keep the display large even in my laptop when it was disconnected from the monitors. It did not have any other effect on how the display appeared on the external monitors. Once I unchecked the override high DPI scaling behavior box I was able to open LR in my laptop (disconnected from external monitors) and it appeared as it should on the laptop monitor.
I have not had this problem in the past. It makes me wonder if there was some kind of software change with the latest updates from Adobe. This situation is making it nearly impossible for me to work in LR on anything but my secondary monitor.
Windows display scaling controls the size of the text in the top menu bar. The LR option Preferences > Interface > Font Size controls the size of all other text. Try setting it to Small or Automatic and restarting LR.