Use the Select All command to select all pixels on a single layer or in the entire image if the image has only one layer. The Select All command selects all pixels on the image canvas, including pixels that have been moved off the canvas.
To select all pixels in a layer or an image:
If the image has more than one layer, on the Layer palette click the name of layer.
Choose Selections > Select All or press Ctrl + A. A marquee appears around the layer on the canvas.
Making Selections with the Freehand Tool
Making Selections with the Magic Wand Tool
Making Selections with the Selection Tool
Selecting All Non-Masked Areas