I have searched for how to set a custom resolution for Minecraft since the 1.6.4 modpacks came out. All that I found in this forum is that it doesn't work with 1.6.4.
If you go to the Minecraft folder for the modpack you play (for me it is C:\Users\********\Desktop\FTB\Direwolf20_1_6_4\minecraft) there is a file called options.txt. Inside of it there are two lines that allow you to set a custom resolution:
overrideWidth:0
overrideHeight:0
Using these to set a custom resolution works. You still can't start the game auto-maximized
I hope this helps.
If you go to the Minecraft folder for the modpack you play (for me it is C:\Users\********\Desktop\FTB\Direwolf20_1_6_4\minecraft) there is a file called options.txt. Inside of it there are two lines that allow you to set a custom resolution:
overrideWidth:0
overrideHeight:0
Using these to set a custom resolution works. You still can't start the game auto-maximized
I hope this helps.