[Feature] resize exported image height

[z4hyoung]

Hello!
For feature "Export Image", we now(1.9.4) can config image width, but can not config height.

But sometimes, my document is not large, when I export the document as image, I got an image with large blank as below

Can we remove this blank part from image? Or can we introduce a new configuration item, (e.g. margin) and export the document without too much blank?
Thanks.