Please note that CallMeVilla got the info backwards. The warming mats go in first, then ditra over.
If everything was done right and still in good condition, 5/8" @ 16" oc meets minimum standards. (thank you cheap-*** builder for using 5/8")
You should be able to figure what the span is for the joists, you don't haveta see them to know where they are. You can use regular Ditra, but if you can find it, Ditra XL is preferred.