Okay. If you're just trying to level out the floor versus strengthen it, all you would need is something like 2x6's, but you'd want them to span wall to wall, so that the subfloor would be fully supported. It will be fully supported so long as ONE of the two-- the original joist, or the sistered joist, hit the bottom of the subfloor when laid down flat (the operative word there being FLAT).

As for order of materials, If you're just doing a normal ceramic floor, you want a layer of 3/4" exterior grade plywood, then either 1/4" cement board or 1/8" Ditra, and then your tile. NO tar paper, unless you're going to do a mudset. Figure on 3/8" for the thickness of tile and thinset, and if you need any height past what these layers would give you, then my recommendation would be to do it in plywood for anything over 1/4", and for under, switch up with your underlayment by using 1/2" cement board, or something along those lines.

To determine if the floor will be properly supported requires information regarding the span and joist spacing. Once that is known, Google "deflecto" and use their calculator to get a good approximation. It sounds like you could make up the thickness with wider joists, if I'm understanding correctly.

1-1/8" thickness of plywood is generally the recommended minimum, which can be achieved by screwing 3/4" down to the joists, then a 3/8" or thicker screwed to the 3/4" on top of that (with staggered joints). Then lay down the 1/4" cement board on a bed of latex modified thinset, screw to plywood, and then tape and mud the joints with more thinset.

Good prep work will make all the difference.