Lets think this out.
Traditional hiking boots are designed to:
- Provide support for loads
- Protect feet from obstacles
- Be able to bulge on the sides with increased loads
- Be water proof
When we talk about boots supporting loads, it is about wrapping the boot around the foot like athletic tape, while be able to flex some. It is often recommended that the "free" space in front of your toes be about one thumb nail width, which for me is about 5/8" of an inch. Also before lacing up the boot you need to be able to push your foot forward all the way to the front of the shoe and place a finger between you heel and the back of the boot. You should be able to wiggle your toes with the boot laced, and your toes should be compressed from the boot. So this fitting does not provide a whole lot of variance if your feet swell from long hard hiking.
Because boots are stiff and and not very flexible, most need a break in period. This allows the leather to stretch some, and for your feet to "toughen up."
Traditional running shoes are designed to provide support too, cushion the bottom of the feet from the pounding of feet during running (much more brutal on the feet than walking), and force the foot to an unnatural position to compensate for pronation. Trail running shoes add additional protection from rocks with a bottom rock plate and often "armor" on the sides and front against rocks and tree roots.
Minimalist shoes are only designed to protect the bottom of the foot from impact. Most have a rock plate. XC racing flats only have a minimal midsole cushion -- no rock plate.
I have found minimalist trail runners to have a much stiffer construction on the body of the shoe versus a XC flat. But they are pretty flexible. The flexibility allows you to tighten the shoe on downhills, without constricting the foot and cutting off blood supply. With XC flats, the shoe body is almost paper thin and you would be hard pressed to tighten the shoe too much. However for long runs or hiking in difficult terrain, XC flats increase your chances of impact injuries. There is a significant difference between something like a MT110 and a Wave 4 Universe in this regard.
Typically going up one size increases the length of the shoe 1/6" and proportionally very little in width. Two sizes increases the length 1/3 of an inch.
Longer minimalist shoes are pretty easy to keep the heel stable, even with a longer shoes, assuming the basic last is compatible with the structure of your foot. My feet are fairly narrow and I am thin, so "D" width fits me well around my arch area and heel, but is extremely roomy in the front.
So going up two sizes in a minimalist shoe is going to add very little extra width. Additionally, with any light shoe your feet are going to absorb more impact that with a boot since there is less structure and cushioning. However, when running your foot strike and mechanics are going to change some versus normal running shoes. Your legs are going to absorb more of the impact, and act more like a coiled spring. This is why people experience very sore muscles when switching to minimalist shoes the first time.
Since the front of a longer shoe is going to be a little more spacey, most people will have concerns with the foot moving around and causing blisters. I have found this is not the case for me. But the bottom of my feet are not soft since I am on the trail a lot and walk barefoot when around the house or going for walks.
So my running/hiking shoes are light, flexible and easy to adjust the tightness using laces so my feet are not constricted. The front of my feet have a minimum of 1" free space.