In Shotcut you would need to make a custom video mode for your video before you start.
This is where you initially set up your video mode, before importing video or images.
As far as Inkscape, you would have to be the one to determine the size, but you’re not real clear of what you’re asking. What ever size you want the image to be, it’s best to be at the same size or larger (width, height, or both) than your set video mode. With the Size & Position filter, you can resize the image to your liking.
Example: 600x600 video for Instagram.
Custom Video Mode of 600x600 (1:1 ratio)
(V1, V2, V3 etc… are video tracks)
- V1: Main video
- V2: Image (Any Size)
You can change the size & position of any image with the filter: Size & Position.