If you want “raster graphics”, in “batch mode”, you can directly create a canvas with any size (in pixels). Otherwise, your canvas’s windows size is basically limited to the size of your display (in pixels), but you can use the TCanvas::SetCanvasSize method to set any size (scroll bars may be added to the canvas’s window).
If you want “vector graphics”, you may want to play with the “PostScript output” (you can “cheat” in ROOT and increase the “paper size”, then “shrink” it outside of ROOT):