support advanced keyframes (like gain/volume does)
allow scaling both w and h (size width and height), keeping the same aspect ratio. This probably also means that decimal number should be supported for w/h. This probably also means that a new field might be needed for a % value that modifies the w/h value (w/h values could be hidden by default with a checkbox showing them in the rare case where they need to be modified individually).
provide a drag interface (probably not a slider, as there is no max/min value, unless a slider can be made to adapt to new max/min). Ideally user can click and drag the value or its label to modify the value. Premiere, Unity, and Blender for example have this.
The scaling should not happen relative to the top left corner but rather relative to the center of the clip (by default). Optionally this “Anchor point” could also be re-positioned (relative to the original clip dimensions).
If you set Size mode to Fill, then the rectangle control on the player does this.
The rectangle control on the player provides this.
If you use the rectangle control, then it should only scale using the point you are dragging. If you are using the size numeric widgets, then the position numeric widgets should not change automatically - that would be a disaster. There is no “scale” field like you show in the Premiere UI; there is in the Rotate filter. Basically, our Rotate filter already supports 2 - 4 in your list. Scale is a part of Rotate because often rotation creates black corners, and you may want to remove them in the same step.
Column A shows what I described. Modifying one of scale or width or height would modify the two other two automatically in row 1.
Column B shows a percentage-only size option which I personally would prefer.
Column C shows a new side-note (hiding fit options when they are ineffective) to reduce clutter