He looks back and forth between them, thinking someone ought to intervene with the arguing— no point in any measuring tape comments since 1) they've got the same outward body and 2) everyone here already knows the size. Okay, he doesn't have actual knowledge that Rose knows, but he's certain that between her and the Doctor and all that pent-up want, they must have done something now that they're together, and that's a pleasant thought isn't it—
Right, right, the argument. Focus.
He's about to interject and question if the Doctor... the second Doctor... the one not in the suit means Jack or some other person who got stuck in the past. Seems like a bad idea to comment on it now, and anyway, the two of them are going back and forth so much that it'd be hard to get a word in. He nearly puts himself bodily between the two of them (focus, focus) and takes a step with that intent before he stops there and turns to look to Rose and Ianto.
The look on his face is pleading for one of them to do something to intervene, because Ianto is much more reasonable than Jack and Rose is much better at positively appealing to emotions. He might accidentally cause a fistfight to break out if he gets in the way.
Really, he should get a prize for managing to keep his mouth shut.