The gauges are smaller because of much higher PPM in this monitor.
To correct this you need to open the "settings.ini" file in SimVim folder and enter this line:
DPI = 100
( it's your monitor resolution - 100 pixels per inch, or something close to this)
Or, instead you can add this line:
Screen_Horizontal = 487
(If you screen width is 487 mm, if not, enter what is yours in mm)
Note, that the instruments positions is adapted for the screen with 16:10 ratio, not for 16:9, so some gauges (e.g. engine) can be not fit the screen height, because the height of the 16:10 screen is 297 mm, and 16:9 height is about 275mm.