If no solution has solved your problem, then I suggest using regedit.
Doing so requires administrator privileges. Use Win + R to open the run prompt and then type in regedit to open the software quickly. The path depends on your OS. I am only familiar with the 64-bit.
HKEY_LOCAL_MACHINE > SOFTWARE > WOW6432NODE > Gravity Soft > Ragnarok
From here, you simply double click the values after Height and Width and use the Decimal value to set it easiest. You can also set ISFULLSCREENMODE to 0 (for off) or 1 (for on) depending on your preferences. Hope this helps.
Note: Don't have the game running while you're doing this.
