In r7609 in the JA2.ini file (in the main game folder) you can choose from the following resolutions:
Toggle Spoiler ;*********************************************************************************************************************** *******
; You can choose from the following screen resolutions.
;
; 0 ... 640x480
; 1 ... 960x540
; 2 ... 800x600
; 3 ... 1024x600
; 4 ... 1280x720
; 5 ... 1024x768
; 6 ... 1280x768
; 7 ... 1360x768
; 8 ... 1366x768
; 9 ... 1280x800
; 10 ... 1440x900
; 11 ... 1600x900
; 12 ... 1280x960
; 13 ... 1440x960
; 14 ... 1770x1000
; 15 ... 1280x1024
; 16 ... 1360x1024
; 17 ... 1600x1024
; 18 ... 1440x1050
; 19 ... 1680x1050
; 20 ... 1920x1080
; 21 ... 1600x1200
; 22 ... 1920x1200
; 23 ... 2560x1440
; 24 ... 2560x1600
; 25 ... Custom Resolution
;*********************************************************************************************************************** *******
; The screen resolution of the game
SCREEN_RESOLUTION = 5
; The screen resolution of the map editor
EDITOR_SCREEN_RESOLUTION = 5
; Applicable only when set to Custom Resolution (25).
; Make sure monitor supports the custom resolution if playing in full screen mode
CUSTOM_SCREEN_RESOLUTION_X = 0
CUSTOM_SCREEN_RESOLUTION_Y = 0
I imagine it's the same in newer releases of v1.13.