The back piece is 7" in diameter with a 2.5" hole, the same dimensions as a standard 45. I recommend lightly sanding the tabs to help the back slide into the base easier. 

The photos are slightly different than the model, after printing I found a couple of things that could be improved: 

1) I scaled the letters down slightly to allow for an easier fit, they will likely need to be glued in but I had to sand and apply a lot of pressure to get them in as it was.
2) I removed the inner portions of the letters from the base, I found it difficult to fit the letters over them and I don't believe this will affect the look of the final assembly but it will help it go together much smoother.
3) The stand was a little unstable so I added a draft angle to the back of the base extending it out a little further. I am not going to bother printing a new one for myself as it stands just fine, its just a bit too easy to knock over if bumped especially in the case of a double album where a record will still be in the sleeve. If the stand is still unstable I recommend hollowing the base a little and adding some form of weight.

All of the labels I could find with the 2.5" hole have the title of the song on the bottom so I made a custom one, I included a pdf in case you enjoy the pun as well. You only need half the label, I just included more to ensure no white space remained after cutting. Tip for cutting a good circle: use a compass with a thin hobby knife where the pencil goes. (Disclaimer: This print and myself are in no way affiliated with Columbia or Simon & Garfunkle)

I recommend printing the back with a concentric top fill pattern, its not perfect but it helps to give the illusion of it being a real record.

I've been having some issues with bed adhesion hence the funky lines towards the top and the lifted corners of the base.