If I understand right, you would like to change the size of your figure in the report with a variable.
If I understand your question correctly, you may be able to do it like this. Save the file, make an image object with the file, then change the height and width with a variable.
h = figure;
print(h,'imageFileName','-dpng');
imgObj = Image('imageFileName.png');
widthStr = '6in'; % this could be a variable set in the workspace
heightStr = '3in'; % this could be a variable set in the workspace
imgObj.Height = heightStr;
imgObj.Width = widthStr;
append(report,imgObj);