I tried FFG Standard American (57 x 89 mm) sleeves. It's a tight fit and the sleeve appears to bend the card very slightly (approximately 2 mm arch). You could use them in a pinch, but if you want your cards perfectly flat, I'd look for something just a little bit wider, maybe 59 mm in width. I haven't tied Mayday or TMG sleeves.
I sleeved using the FFG green and have noticed the arching as well. It is probably better to find a slightly bigger sleeve, but I have already sleeved all the cards and feel although extremely snug, it works well enough for me. Flattening the sleeved cards over time seems to resolve the bowing.
I would like to add that my cards seemed a tad bit bowed out of the box. I'm not convinced that it is entirely the sleeves causing it.
If you take this route of sleeving your cards, I warn you to take it very slowly and use extreme care while you sleeve so you don't accidentally damage your cards as the fit is button popping tight.
I took some cards out to check for any damage from sleeving and didn't find any signs of any damage done to the cards.