Alex,

A late response. I have been (more or less) in the position you are in now. It worked out fine and more than fine. You've already had some good advice (as well as a little of the reverse, sad to say).

It might be good to give her an idea of what she has to expect; she must already know you are big, but probably not how big. Pick a suitable moment and take her hands in yours so she can begin to get to know you more definitely. It is better that way than what might be total shock at your size when she sees it for the first time.

Also you can reassure her when the time comes that you only intend to put a small fraction in ... you will know that she has to be comfortable with your thickness before you can begin going in to any depth at all.

She is a lucky woman, but no doubt she already knows that.