Anim GIF Studio is a shareware software. Unregistered version is a fully functional 25 day trial version. If you like this software please register.
How to resize an existing anim GIF?
GIFs are limited to 256 colors.
Resizing an existing anim GIF happens in full 16 million colors (24 bit) mode, and the result is converted back to 256 colors when saving the anim GIF, so it is very high quality in Animm GIF Studio.
Select menu: "File/Load GIF File..."
In the upper-right part of the main window the original dimensions are displayed
Set "Saving options": "Frame width" and "Frame height" to the needed values
Checkmark "Saving options": "Resize to these dimensions"
Enter or select an output file name at the top and click "Save GIF"
Note that aspect ratios of the picture frames are always preserved, so it's not possible to "stretch" the pictures.
Importing video frames
Importing video is sadly not frame accurate, it seems ffmpeg.exe does not always return the video frame pictures precisely.
Just set a little bit bigger interval than actually needed and after the import completes remove the unneeded frames.