In addition to being honest with her up front, I would also emphasize to her that, if she chooses to come with you, she is merely there as an observer. That She isn't there to play any role is the decision making process (i.e. no complaining about the size!). If she can accept that before even going into the shop, the tattooer won't have to deal with any sort of spousal disagreements, which is good because, let's be honest, that shit is awkward to witness.