There are a few ways to select everything in a drawing:

 

1. Set a gesture to Select All. Go to Settings > Gestures and set either a two, three or four finger tap to "Select All". Go back to the canvas and use the gesture to select everything on your canvas.

 

2. Zoom out and use the Lasso tool to select everything in the drawing. Make sure "All Layers" is active in the Selection menu at the bottom of the canvas. This only works if your drawing can fit inside the screen while zoomed out to the max (5%).

 

3. Use Layer Selection to select all layers. In the Layers Panel, tap on the active layer to open the layer options pop-up. Tap on the selection arrow icon to select everything on the layer. 

To add more layers to the current selection, tap once on another layer in the menu to select it. Tap on it again to open the layer options and tap the selection arrow to add the contents of the layer to the selection. Repeat until all layers are selected.