Resize a Manipulate by grabbing a corner

Does it fit your needs?

magnifier[expr_] := DynamicModule[{initImageSize, imageSize, mag = 1}
  , initImageSize = imageSize = Rasterize[expr, "RasterSize"]
  ; Pane[
   Magnify[expr, Dynamic@mag],
   AppearanceElements -> "ResizeArea",
   ImageSize -> Dynamic[imageSize, {Automatic, (mag = Mean[imageSize/initImageSize]) &}]
   ]
  ]

magnifier@Manipulate[Plot[a x, {x, -3, 3}], {a, 0, 10}]