The size - pixel height x width entirely depends on the model of the iPad or iPhone you are using. The higher the screen size, the bigger is the canvas. In other words, a drawing made with an iPhone is smaller than a drawing made with an iPad.
At this point, the app does not let you set specific pixels height and width, for a canvas. However, in case you are in need of such a feature, please raise your voice so we can make the next feature planning accordingly.