This is a limitation of the WinForms grid used by the Live Variables control. The Expression column will automatically fit the space remaining after all other columns. Please try resizing the other columns instead and the Expression column will get automatically adjusted.
The Visual Watch and Live Variables windows are still using the older WinForms technology (other windows have been converted to WPF) and hence they indeed may look distorted on the high-DPI displays. We are planning to convert them to WPF in the next major VisualGDB release. That said, it should not affect any functionality, i.e. simply double-click underneath the header and start entering variable name. The watch item will be created as expected despite the incorrectly looking header.