The vintage green tiles in the first picture are lovely. The darker ones below with the bevelled edges are just so wrong in the setting shown. I am also a lover of green but you have to get the right tone and keep the shape as smooth and flat as possible. I am a big believer in having just one standout element of design in a tile - either its unusual in shape with a matt finish or a special colour with no fussy edges.

Google glass subway tile and see if that's what you're after. Beaumonts have them.

I think a dark grey grout would look great, the wider the gap the more the tiles stand out, if it wouldn't cost too much and you had some spare tiles you could do a mock up on a piece of board , sometimes the only way to really gauge things is to actually see it

Hi Emma. I'm not sure about shape of tile giving impression room is larger, but I do think large in either shape is the way to go. There are likely Houzz Stories which address this topic if you want to do a search. May I ask which pattern you have in mind for the subways? If there are other tile stores in your area, I urge you to visit them as the advice and helpfulness in the customer service can vary considerably.