To be honest, you'd get a cleaner and quicker result by tiling over with bigger tile. Small tiles are going out of style anyways...

That's what I would do. Roughen up the remaining tile, apply the correct tile on tile adhesive (very important!), retile and grout, done. 5 hour job max, from the look of your bathroom it's kind of small.

I'd not do that if it was a bigger bathroom / area (stress fractures more likely).