tankless water heater size recommendations

We are about to embark on a bunch of work and I was hoping to get a recommendation for a tankless water heater...particularly, what size should we get!? This is NOT for a dual purpose tankless that works for the radiant heat, as our boiler is just over a year old.

If this helps:

We have a 4 br. 2 ba Eichler...1724 square feet. There are 2 adults and 3 children in the house. Our monthly water usage is about 6 CCF's.

I was also wondering if anyone has mounted their tankless outside the garage area rather than inside? If so, have there been any issues with that? We are going to be replacing our exterior panelling and would have to coordinate the piping and panel installation into the project should we mount it on the outside. The reason we want to do this is to keep more space available inside the garage for other use/shelves as the garage is a single.

Thanks for any and all help. My email is in the profile page should you prefer to send response directly to me.