I got a 32 gb micro sd card for 3ds games because I like to have lots of options available without swapping roms. 8 gb micros are fine,but just know most games are .5gb to 2gb and some are 4gb if you don't trim the roms. I never trimmed my roms just because I didn't have to. Most of the staples are 1 to 2gb like pokemon kingdom hearts fantasy life animal crossing kirby most mario games etc. So at most you can have like 8-16 high quality games, and at worst 2 4gb games, but those are rare like donkey kong i think. You are already saving so much money not buying the games. I would suggest getting a 32gb class 10, or at least 16 gb for convenience. The sd card is fine at 2gb but I would at least get 4gb there I'd say to just have more than enough space. If you do plan to have .cia installed I'd go bigger. Personally I keep everything on my microsd cards so I can't offer much advice there. I just feel like installing .cia and everything is more work than it is worth if you have a big micro sd card. .cia is important for dlc and updates though so 4gb might be a good baseline to work with if you have the option but I think most dlc is small enough that a 2gb would work.
This is a link to the rom tool discussion
http://gbatemp.net/threads/3ds-rom-tool-rom_tool.349314/
You can turn 2gb games to like 1gb I think. I have never used it.
Edit: I think most DS games are less than 1 gb. .5gb or less is very common if i remember. 8gb would be more than enough for those if I remember correctly.