Use any sized beet you like and make. They shrink in size, so bigger beets will make a nice size chip. The seasoning is totally up to you - adjust it the way you like. I prefer using Sea Salt.
If you have a convection oven, you can make 3 pans of these at one time.
2 large beets
2 tablespoons olive oil
1/2 teaspoon sea salt
1 teaspoon freshly ground pepper
Preheat the oven to 300° F. (or 275° F for convection oven). Line two or three baking sheets with parchment paper.
Peel the beets. Slice them to about 1/16" inch (1.6 mm) thick, at the most. Toss them in a large bowl in the olive oil and salt and pepper. (If you have time, you can wait for 30 minutes or so after this step - especially with the red beets, the salt will draw out some excess moisture). Lay them on the baking sheet, just barely touching.
Bake for about one hour until the edges are crisped up and most of the beets have dried out. Because you will have different sized slices from the same beet, you will have some smaller chips and some larger chips. This will let you have a little bit of variety in the crispiness of the chips and you can decide how you like them for your next batch.
Remove from baking sheet when you think they're done and place them on a serving plate on in a bowl. They crisp up a little as they cool down. They are fine over night at room temperature or you can refrigerate them for a little longer storage. To crisp them up again, just put them on a baking sheet and bake them at 400 ° for a few minutes.