Sandpaper letters and numbers are a key Montessori material. By the child tracing the letters with his or her finger it helps solidify the shape in the brain by using kinesthetic , tactile and visual senses. These are time consuming to make yourself and are rather expensive if you purchase the Montessori version. I have this set and I love it (though we’ve decided to teach cursive first I’ll still be using the number tiles). It includes both upper and lower case letters, numbers and symbols. They are grooved in the wood and are amazing quality, very sturdy. I actually really like the grooved ones as they help the child trace a little better. (click photo for link)
Here’s some photos of ours so you get a sense of the size. They’re a really nice size. Not too big and not too small.