I'd find 1" close in color ( 1" comes attached to 4", 6" and 12" mesh backer) or use 4" in mixed color. So that you walk away with something that makes work go faster plus do a better job. Use a 2 or 3 foot straightedge bridging from old tile to other old tile to press new tile into thin set perfectly level with one another as well as old tile. An aluminum carpenters level makes the perfect straightedge. Be sure to apply thin set with notched trowel.

Thanks for the tips, folks.

I started working on this last night, and there are some complications.

The tile isn't really 1x1--it's smaller: 29/32", square. (That's not going to matter anymore, though.)

The rot is more extensive than I'd hoped. It extends to the back wall (and probably to the right, under the vanity. The walls consist of old tile covered in acrylic (?) sheet that covers the whole lower half of the wall, so the tile edges tuck under the sheet. That will make the walls harder to interface with.

In the course of home ownership, I've done lots of DIY work, but never tile, so I'm not sure what's normal. However the existing tile attachment to the plywood is not quite what I

expected. There's very little thinset attached to the tile and none attached to the plywood when I remove the tiles. As a result, I'm finding that large sheets of the tile can be lifted, as of they're attached to each other with grout, but barely/not attached to the plywood.

I don't think any of the existing tile will be salvageable and possibly none of the subfloor.

I guess the upshot is that I won't have to match tile, since I'll be starting from scratch.

First find out where the water is coming from that's rotting out everything.