Yes, I would think that a 58 isn't all that big. I think the "average" frame size is somewhere around a 54-55. You can always ask Trico anyway or ask a LBS, they probably ship bikes for people all the time. For a MTB, you might have to take the handlebar out completely, because they are longer than drop bars. Might want to make sure you get/buy a stem with a removable face-plate so that you can take the stem off without having to remove all the components.
As far as how long it takes for me to pack, it depends how rushed I am, I mean, as long as everything comes appart smoothly, I would say the fastest would take about 15 minutes but I would usually when not rushed take about 1/2 hour. That is because I usually wrap the main tubes with pipe insulation. I will pack other gear into the box that will fit and will fill whatever space leftover with packing material to keep things from moving.
If you do buy the Trico Ironcase used, make sure you get the fork dropout spacers with it. My IronCase that I bought used came with 2 metal "skewars" that you put in the front fork dropouts and the rear dropout to prevent the forks from getting crushed. I believe they are standard when you buy a Trico new so if you do buy one used, make sure it comes with it or if not, make sure you use something similar. Hate to arrive somewhere with a bent forkleg!