Congrats on your new addition!
First, don't put the litter in the crate. To crate train a dog, you want them to learn to hold it while they are in their crate. If you put the litter in there and teach them to poop & pee where they sleep, it will develop bad habits for the future.
Second, I use a crate for my pug that is 24" x 18" x 19". He is now fully grown and it fits him well. You may want to get either a smaller crate for when your pug is a puppy, or get a crate that has a divider than can be removed. The idea of crate training is you want to have the dog have enough space to get up, turn around, but you don't want them in a huge crate that would allow them to go potty on one side and then sleep on the other.
Also, when you have your new pup, assume you'll probably still have to do some potty training and plan to let them out about once every two hours (or walk once every two hours) for the first few weeks. My pug learned potty training very quickly. They are generally pretty smart about it as pugs are a people pleasing breed.
As another side note, not sure if this is your first pug, but just as a warning on the crate training, you will probably want to keep the pug in the crate overnight to get him used to it being his little "den." Pugs really always want to be with their humans, so when we were crate training our pug it was hard to get him to go to sleep in the crate at night the first week or two because he would bark - he wanted to be in the bed with us. He now goes into his crate (in our bedroom) at bed time with little problem. I wouldn't recommend keeping any dog in a crate more than about 6 hours at a time, however, because they will go potty and/or get stir crazy. And for a puppy, I would recommend not leaving him alone in the crate more than about 3-4 hours at a time, except during the overnight hours.
Hope this helps, and good luck with your new pup!