Along the way, I contemplated my choices for a breed (and ended up not describing Romeo other than the fact that he is big and very friendly). But here were a couple of possibilities:
Image by Tracy Lee
A Saint Bernard:
Image by 3D Photographers
And a Bernese Mountain Dog (a little bit smaller)
Image by StooMathiesen
It takes awhile to decide these things and often I travel down the wrong road and end up dropping the search (and the character) altogether. It all involves a lot of searching...and reading. Here's a good article on The Biggest Dogs in the World - There Be Giants. I hope I can keep Romeo in the book, but he may have to go. Too many characters can spoil a romance.