you got it...wrong.
IF EEPROM = YES THEN SAVE INTO SKY3DS CART (this cannot be accessed by us, at all, short of taking apart the sky3ds cart and getting a eeprom reader)
IF EEPROM = NO then it saves onto the ROM. the rom however is on the micro sd card, so it does technically save to the micro sd card.
also, fantasy life uses normal saving. you can treat it like a normal 2GB 3ds game. (most 3ds games a 2GB or lower, even monster hunter)
next, you get a MICRO sd card, not a sd card. they are very small. usually wherever you buy them from will also give you a micro sd converter for free. by default they give you one that convers into a sd card to put into your PC, but i prefer a micro sd to USB converter, as not all PC's have a SD card slot, and i just prefer USB overall.
as for space, get ATLEAST the 8 GB, it would only have very few games tho, like 2-3. the 16GB would be alot better, holding about 7. 32 would be best, but they tend to be pricey, still they would hold everything you want. personally i'd say TWO 16GB micro sd cards, but if you never had a micro sd card before, one would be better, as with two its easy to lose one of them on axident (they are really fuckin tiny)
last, your first 10 EEPROM = YES roms will save onto the sky3ds cart. neither the red or blue button have more then 10 YES EEPROM save slots. the blue button will auto change EEPROM YES to EEPROM NO by itself, so its less work for the user, the red button does not.
for ALL carts, red or blue, whenever your using eeprom=no, here is the compatibility chart i made so far. the compatibility is based on the template, and the sky3ds team releases new templates every few days with bug fixes. adding more and more compatible games. an example is two days ago persona Q and the harvest moon games were added as playable, except for one of them.
https://docs.google.com/spreadsheets/d/1ZUSaCSxa8M-wPfvHtnlnJGAqjRsxkPa4hsuNCIjkLOE/edit#gid=0
PS: 4GB roms can work fine, however some require RED button users to change the template a different way then they normally would. however for blue button users, because the cart auto changes the eeprom status for you from yes to no after the cart runs out of save slots, BLUE button users do not need to worry about this. either way, 4GB roms work fine on red or blue.