As your documents begin to contain more and more layers, sometimes those file sizes will grow to be quite large. To reduce the file size, we can merge layers or flatten images. These operation are destructive and remove some of the flexibility.

If we have any layers, that extend beyond the canvas area, we can crop to reduce the file size. Follow below steps to crop

  • Go to Select menu and choose All, to select whole canvas.
  • Go to Image menu and choose Crop. Document size should be reduced now.
  • To de-select, go to Select menu and choose Deselect

This article discuss about merging and flattening layers in Photoshop.

Merge Layers

I would suggest that you merge layers that don’t overlap. If we merge overlapping layers, we cannot unmerge them to move them independently of one another. To merge

  • Select layers to merge using Layers Panel.
  • Go to Layer menu, and choose Merge layers.

Flatten Image

Similarly for flattening an image,

  • Select layers to merge using Layers Panel.
  • Go to Layer menu and choose Flatten Image.
  • And that will flatten all of the layers to a single background.

This operation is destructive.