What are the dimensions of the tank? That could give me a better understanding of what size you need, if its just the standard 36x18x18 then 16x16 tiles work, leaving a small gap in the middle or on at least one side or 18x18 tiles work, whichever is easier to find.
Make sure to go for rough tiles and not slippery ones, also if your worried your beardie would be freaked out by the sudden change trust me they wont, or at least not too freaked out. When my juvenile walked on her dig area for the first time she was confused somewhat and couldnt help herself but dig around, though since the first day she hasnt dug at all. She also used to be on paper towels but is now on mostly slate tile and she didnt get freaked out about it.