I think they make tiles that are 4.25 inches or so, which would be about right including the grout.

I'm sure one of the tile guys would have a better or more definitive answer, but at least you can look for tiles that size in the meantime.

You are leaving the yellow tile? If you can't find matching tile or a coordinating decorative tile to use, I would look for a taller mirror and hang it so it covers the holes. I think unless you use a whole row or two of different tile that just two in those spots is going to look odd. You could space several of them the whole length of the vanity, or find a mosaic glass tile with yellow and black to try to tie it all together and put a few rows of that the whole length of the vanity. Or even take out the tile above the backsplash and use all granite to the mirror.

But that's all going to look like a hodgepodge. I think your best bet is a bigger mirror.

I can't tell how far you want to go with this - beyond the vanity? Gray is a color that goes well with your shade of yellow, so is there any chance you can change the floor and paint the walls? Painting the vanity could also help.

Here's a picture from a website featuring lots of colored bathrooms:

Here is a link that might be useful: Color!

Thank you all. I am just hoping that the backsplash fits under soap dishes well.
I thought about many options including some accent tile or a bigger mirror (mirror sounds good but tile is and drywall not even so it will not be pretty. I wish I just rip off tiles and replacing all but that's not easy as well.
Thank you again!