I was just looking for the same problem, and I found these regents very useful for achieving my goal:
BlueStack Screen Resolution Setting
Resolution can be changed by editing the BlueStack registry:
HKLM \ SOFTWARE \ BlueStacks \ Guest \ Android \ FrameBuffer \ 0 \ Height
and
HKLM \ SOFTWARE \ BlueStacks \ Guest \ Android \ FrameBuffer \ 0 \ Width
found this information here
To make sure this matches my tiny screen exactly, I went to see my screen resolution
and then I copied this information to these addresses through regedit.
tip: make sure you use decimal values ββwhen pasting permission.
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