The tubes were cut 5mm oversize to allow for easier soldering. It doen't
matter if they're slightly too long as the next picture will show.
Although I live in a metric country a lot of our copper pipe is supplied in imperial (inch) sizes so it can be confusing. I figure near enough is good enough.
When I build, I allow as much tolerance as possible. I don't have a machine shop, so a lot of things are done by hand and eye. The more 'fudge factor' the better as it allows for slight mistakes. No "1% tolerance" with me. But if you're careful, you can do pretty neat work.
I used water inlets and outlets on the side of the firebox for convenience instead of the top and bottom. You could use a couple of the inside tubes for this job, just cut them longer and adjust your design.