I would actually wait on buying an micro SD card for the Switch until you need one. Unless you're planning on going 100% digital, then you probably should get one immediately, but if you're planning on just going with where is the cheapest place to buy games then you should be able to last a while without upgrading.
With Micro SD cards dropping in price every year (I bought a 32 GB one at Costco for $20 in 2015 and just a month ago bought a 64 GB one from there for $20), I would wait until you need it.
I recently moved around my Wii U games to a smaller hard-drive and I think it's a decent starting point for what to expect for how much space you'll need in the future. I expect on average for Switch games to be a bit bigger than Wii U titles, but it's a good estimate considering the similar sizes for Zelda: BOTW on both Wii U and Switch.
My Wii U Storage Situation:
21 Wii U Disc Games (save games, patches...) = 6,659 MB
13 Eshop Retail Games (Disney Infinity, Donkey Kong, Wonderful 101...) = 78,688 MB
18 Eshop Only Games (Runbow, Child of Light, Guacamelee...) = 14,080 MB
10 Virtual Console Games (Mario 64, Link to the Past, Super Mario World...) = 279 MB
2 Wii Eshop Games (Metroid Prime Trilogy & Mario Galaxy 2) = 8958 MB
5 Console Apps (Netflix, YouTube, Animal Crossing Plaza...) = 858 MB
Total Space Used = 109 GB
Console Space Left (after internal operating system, Wii mode etc...)
32 GB Internal Storage = 4.6 GB left
128 GB usb Storage = 29 GB left
After 4 years of Wii U ownership I really only needed to add a 128 GB usb to my console.
I'm thinking that a 256 Micro SD card is going to be the one that will last me for quite a while on the Switch and something tells me that by the time my 32 GB is filled up they should be considerably cheaper than they are now.
