Theres literally nothing you can set.
I don’t use this feature and I forgot just how poorly its designed.
Unfortunately, when you create videos from images. Video size is based on canvas size. Which is tied directly to your monitors resolution. Image size, Aspect ratios, etc.is a 3rd factor.
My 1920 x 1080 monitor creates a 1408 x 792 canvas area. My 6000 x 4000-pixel image is rescaled down to 900 x 600 pixels in the image below. The re-scaling is less than Ideal.
If I take SnagIt over to my 2560 x 1440 monitor. The same image is scaled to 1500 x 1000 pixels.
None of this is ideal or worthy of the original imagery.
Smaller images are lost in an over scaled background as you are encountering.