As you probably know, every piece of wood is different, so dimensions are for starting purposes only.
Also, some of this depends on how large and at what angle your recurves will sit, and it would help to know the width of your stave.
Handle: as short as you can manage, or possibly, since yew bows generally allow a thicker cross section and often crowned bellies, maybe even a bend in the handle bow. BITH bow isn't a bad idea because of that, and that 61" is none too long for a 27.5" draw length, and that you are losing some limb length to the recurves already. Narrow handles help and BITH bows have a narrower handle by default.
Fades, as short as you can manage. If the limbs are only barely wider than the handle they can be pretty abrupt, but if the limbs are wider, make them long enough to guarantee a smooth transition in both planes.
Thickness is what you end up with and partly depends on design and partly just on width. The limbs should retain as much width as possible up to the recurves to prevent instability, but the very tips could be 3/8" x 3/8".
I'd try to rough out maybe 1-1/8" square handle area to start. Obviously, wider limbs can start thinner, but a BITH bow I would rough out tapering back to front to maybe half inch thick tips, and split the difference just out from mid limb.
Remember, bows are only born during tillering, never by formula, but they often DO get ruined during a poor layout and preliminary shaping. Be conservative and always remember you can't put wood back.
