To resize multiple layers together in PSP, you can create a layer group and then resize the group. Here's one way:
Select the lowest layer you want to resize as a group. You need to work from the bottom up.
Select New Layer Group from the top left corner of the Layers Panel (under the word Layer.)
In the New Layer Group dialog box, toggle Highlight in Layers Panel. This helps you see what is grouped.
Now, start dragging the layers into the group in order working from the one right above the group. You'll just drag them under the word "Group."
Once you have all of the layers grouped, select the layer called "Group." Choose Image > Resize from the Menu and uncheck Resize All Layers. Type in the new dimensions for your layer group and click OK. This will resize only the items in your group.
You can do this for all of the pieces you want to resize together, leaving the background out of the group. This will resize just the pieces and not the whole canvas.