I don't think export_fig allows you to set height and width. But, consider using
-m<val> - option where val indicates the factor to magnify the
on-screen figure pixel dimensions by when generating bitmap
outputs. Default: '-m1'.
The number aside m is the factor to magnify the image displayed on the screen. So if you believe 1 to small, increase it. This is taken from the description of the export_fig function. The author made available this option properly because of resizing: