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Hi, you have a lot of questions. :-) I try to answer some of them.
If the input image is smaller than the detection window nothing will be done. See hog.cpp line 1033. If you want to process the small image, you have to resize it by yourself before calling detectMultiScale.
In case that the input image is larger than the detector window. The method detectMultiScale compute the number of scales to downscale the input image depending on the size of the image, the detector window and the scaling factor. See the loop in hog.cpp at line 1030.
The maximum scaling level is limited to 64 levels by default. See: objdetect.hpp Line 596 and 604. You can change the number by putting a value to the nlevels param of the HOGDescriptor constructor.
I hope this helps you a bit