Can go as big as 2tb Micro SD card I believe with the Nintendo switch but from personal experience don't get too big. If anything ever happens to it you will be down financially plus have to redownload your games. Go for smaller not that small I go for 32gb since they are currently so cheap and plan on getting another and just swapping out when needed.
I believe 128 GB is all you'd need. Zelda, Mario Kart, Arms, Splatoon 2 and minecraft all fit on the internal 32GB. So you wouldn't need anymore than 128.