Adjust height of person sitting on couch

You are going to find that after making the girls taller, they are not going to perfectly cover and hide the original.  Remember Free Transform and hold down Shift to makee them taller, and before you OK that, right click and choose Warp.  Then carefully move individual parts of the image to cover the original as best you can.

When done, make a new layer between the original and new enlarged girls layer, and paint over any of the original  girls that are spilling out from behind the enlarged girls.  Alt click with the brush selected to sample the colour.  You don't need to be accurate as the new girls layer will hide errent pixels.  The solid colours of the catroon are going to make this bit super easy.

When you've got a clean selection of the girls, you need to save it in case things go pear shapped enlarging a copy of the girls.  If you Save Selection it will create a new Alpha channel, but the way I usually do it is to fill it with black in a new layer that I hide at the bottom of the stack.  You can Ctrl click to reload the selection, and something I do a lot is to clip to those template layers.  Not relevant in this case, but a good tool to have in your Photoshop box of tricks.

The way clipping works is you right click the layer above the template layer, and choose Create Cliiping mask.  You can see that the brown rectangle 'layer 2' is confined to the shape of the layer it is clipped to.  This is especially useful when warping, which is not easy to get exactly where you need it to be.