there is no way to set exact size, but if you Edit> Add image, your image size will be in ratio of your current image size,
if you zoom in pdf, when you place image, the image will be bigger in ratio
so you can make sure all your image is in proportion in image pixel size
then zoom in pdf at right level, add image in, so they will be all at certain size with same scale ratio
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if above not work well, you can try copy 1 image, and paste to another place, then replace that image, it will maintain same size