Hello,
resize problem is a pretty tough one.

ConvertX doesn't consider the insight of the video, only its total size as it is provided by the input decoder.

So if you have a source in pan-scan but black bar are part or the video, when you convert that to 16:9 for convertx source is 4:3, so will add 2 black bars at left/right, and the result is that you have a tiny image with 4 large black bars on every edge of the video.

We need to provide an extra setting to decide if the video must be resized to fully fit the screen, or cropped then resized to fully fit. More control on resize/crop is in the task list, expect this in a near future version.