You can change the width of the green boxes with legend->SetMargin(x) where x is some float between 0 and 1.
As for the height, one workaround could be to add to the legend one or more histograms without name and symbol; for example:
legend -> AddEntry(h2," “,”")
(notice the space in the name’s quotation marks, otherwise “h2” is shown in the legend)
and then worry about the vertical placement of your entry within the legend box. This of course doesn’t provide lots of control and ultimately also depends on the automatic height calculation from the legend box size, but it’s something