What size lens will I need on a 1.6 crop to do this?

Extension tubes shorten the minimum focus range of lens. If you have a lens with good optic quality, stacking some extension tubes would turn the lens into awesome macro lens. For example, the Tamron 28-75mm is a great lens, sharp and good contrast. When i add some tubes, it works pretty good.

Here are some examples of my Sigma 50-500mm with 68mm extension tubes. All photos are uncropped. The dragons were about 2 or 3 inche long, no problem filling up the frame.
https://photography-on-the.net/forum/showthre​ad.php?t=217879

That being said, I have used a Sigma 180mm macro. Fantastic lens. Sharp like razor, colors and contrast are stunning.

Forget the extension tubes with zooms. That is very compromised combination. In fact, I would skip extension tubes altogether and just go for the 60mm EF-s macro, or the 50mm Compact macro. The Kenko tube set is $170, the 50mm Compact Macro lens is $239 and you'll get better results.

With the exception of Canon's 50 mm f/2.5, prime macro lenses will let you focus close enough (and "close enough" varies with the lens focal length) to capture a subject at 1:1. This means that at closest focus, you'll be photographing a 15 mmx22.5 mm area on your 400D. At that magnification, it doesn't much matter which focal length your lens is - the DoF will be the same for all of them. The benefit of a longer lens is that the angle of view is smaller, so you'll have an easier time getting a controlled background for your shot, especially in the wild.