If you add a “Placer” or “PIP Window”, you can select its size with “Fill Windows/Fullscreen”, “Fit Window/Letterbox”, “1:1” or “Custom”. In case the input aspect ratios (e.g. portrait video from a mimoCall, 9:16) does not correlate with the documents aspect ratios (e.g. 16:9 landscape) all of the size options is sub-optimal.
Therefore please add an additional “Fit Window and Blur”, which would fill the empty space (letterboxes) with a blurred version of the layer signal.
I did a quick preview of the expected result: