A:
It depends on the requirements for that particular phone. Some phones (and I'm not sure why) specify that it can't use an SD card if the memory on the card is too large. For example, my tablet can't use a micro SD card with a capacity of more than 32GB. I don't know if using a higher storage SD card will affect the device's function or not, but I figure it's better to be safe than sorry. If it doesn't specify a limit, then it probably doesn't matter.
As a general rule, you should also use an SD card with a "Class 8" rating or higher, denoted by a "C" with the number inside. For general use, SD cards with a class 2 to 7 is okay, but most devices work better with class 8 to 10. And for high-end devices (like DSLR cameras), SD cards with a letter "U" are best:
Basic SD usage - "C2" to "C7" (for mp3 players, basic cameras)
Advanced SD usage - "C8" to "C10" (smartphones, tablets)
Professional SD usage - "U1" to "U3" (DSLR cameras)