The vast majority of GMT games use the standard "Magic" sized sleeves, the exception have smaller cards such as in Manoeuvre.
My honest advice is to avoid penny sleeves, as they make it harder to shuffle the cards, the corners get snagged. Its a false economy.
Cards by FFG, KMC and Ultra Pro are good quality and cheaper than Dragon Shield, after trying all sorts of brands, I now stick to using FFG sleeves exclusively; good prices. availability, range of sizes and no holograms
Although Mayday did start recently selling penny sleeves, most of their products are on par with the FFG sleeves. For CDGs, I recommend the "Standard" sleeves over the "Premium" ones, as the additional thickness of the Premium sleeves makes them very hard to stack.