For resizing, please use w (width) and h (height) parameters. Both w and h accept integer values. For example, w_250 sets the width to exactly 250 pixels.
You can resize the file by using both the width and height parameters or with only one of them: the other dimension is automatically updated to maintain the aspect ratio.
For example, to resize an image or video to 800x600 pixels, use w_800,h_600