The 100-400 is the longest Canon-made option that is still "portable."
Do you mean physically long or long as in reach?
Either way I handshoot the 600mm for wildlife, but I would say the 500 f/4 II is probably the longest in both areas that I would consider truly portable
I dont consider the 70-200 f/2.8 to be very large or heavy.
LOL! I knew writing that sentence would bring you into the fray.
I meant long in reach. According to TDP, the 100-400 is about 3.5 lb, and the 70-200 is about 3.75 lb with ring and hood. Above that focal length, the lenses get a bit heavier. The 500, 600 and 800 are 7, 8.5 and 10 lb. The 70-200 is also about 8 inches long, but the three telephoto primes are almost 2x as long or longer.
It's too bad that Canon does not offer smaller aperture options at the longest focal lengths like they do up to 400mm. The Canon 300 f/4 is about $1400 and weighs about 2.5 lb while the Canon 300 f/2.8 is about $7300 and weighs about 5 lb. What if they were to design smaller aperture lenses for 500 and 600mm? They would be a lot more appealing at half the weight and less than 1/3 the price (300mm comparison is unfair because the 300 f/4 is a much older design)! Imagine a 600mm lens at f/5.6 and 4.5 lb for $4000-5000!