First just open the required letters in PSE.
Then drag all of them into your required file.
Now we are going to select all the "letter" layers. Hold Shift key and select all the neede layers.
Now with all the letter layers still selected click on Image->Transform->Free Transform
Now there are a few ways to go about adjusting them - you may want to grab the top right corner with your mouse hold the button and drag it down. In order to maintain the same proportions of the letters make sure you check "Constrain Proportions"
Now all the letters are resized - now just click and drag individual layers to place them :)
