ROMs should be the exact size while microSD's and generally any storage have a bit less than the advertised capacity available. With the ROM trimming material (in this section), it could/should be possible to trim ROMs down to fit on a ~3.7GB microSD, won't know until we test.
If you think you're going to use a lot of space, invest now in a bigger card so you don't need to buy a bigger one later and waste more money. I've only filled about 8GB of my 16GB microSD but that's going to change soon XD Glad I got the extra space - I'll need it. Especially when your games are 4GB each, and then you're going to need room for cart firmware files and such.