Look for the `options.txt` file in each modpack folder.
Open it in your favourite text editor and find these two lines
overrideWidth:0
overrideHeight:0
These set the Minecraft window resolution.
overrideWidth:1280
overrideHeight:720
However, if you wish to record your sessions, you will need to take the windows borders into consideration, so effectivly, to get an output of 1280x720, set them too:
overrideWidth:1296
overrideHeight:758
Enjoy.