OK, so we're talking about
TimberStrand engineered wood joists, right?
Engineered wood framing is generally specified to at least meet building code, but one can never be sure. If you're planning to tile over it without first finding out what you've got, I guess we'll just start from there.
If you'll visit the Ditra website and download the
Installation Handbook, you'll find manufacturer recommended trowel sizes there.
As for pre-filling and bonding tile to the Ditra, keep in mind that thinset mortar
does not adhere to the Ditra membrane, it simply fills the engineered contours of the material, sets up, and provides a mechanical bond. The manufacturer approves of the pre-filling of the "waffles" as an acceptable method of installing tiles, but I believe they'd still prefer that you fill and set at the same time. Many pros here report pre-filling all their installations.
My opinion; worth price charged.