Please make sure you are posting in the correct place. Server ads go here and modpack bugs go here
The FTB Forum is now read-only, and is here as an archive.
To participate in our community discussions, please join our Discord! https://ftb.team/discord
I think the feature just might not work, I have tried several times myself to get it to change the resolution when it starts up with no luck. What I have been doing, which is manual. Is just dragging from a corner :/.
There's a program called Sizer that would let you resize windows to an given resolution. You could use that since it seems as that feature for the launcher don't work.