Like Marianna says, size up a half size. I don't know if this is unique to Me-too though. I find that for ANY flat, I have to size up. That's because with a flat your foot has little arch support and as you walk, your foot flattens out and gets wider.
My experience with this brand is that each style fits differently. Some styles seem better designed for narrow feet, and others for wider feet. I have a wider forefoot that is kinda high, sounds similar to your feet. I have not tried either of these styles but my most comfortable Me-Too's are the Nini's, which look a lot like the Nadia's. They may actually be the same shoe but the Nadia's are the most current rendition. I couldn't be sure unless I tried them on.
Assuming they are the same, I did size up 1/2 size with the Nini's and needed to put a piece of moleskin in the back of the heel to keep them on my feet because that made them just a smidge too long (though the smaller size was definitely too small). The moleskin is still there but has worn down so much, I don't think it's doing anything anymore. At any rate, when I first put them on, they are a bit loose (cross your legs and the shoe threatens to fall off), but after just an hour or so, as my feet flatten out, they fit perfect and no more falling off. I walked all over Paris in these for 8+ hours a day, over a 10 day trip - no blisters! That's how comfortable they are.
Swann's - never worked for my foot. I think they are better for a more narrow foot?
I also have the Legend's. I find they are quite comfortable, except when I wore them in Hong Kong, they weren't quite as good as the Nini's. The problem there was the high heat and humidity: my feet swelled so much, the elastic around the shoe dug into the top of my foot by the end of the day. The Nini's ultimately were better. But at home, with our lower humidity and heat, I can wear the Legend's for 8+ hours, no problem.
