I did not have a swap partition/file because my USB stick is slow and that'd just drag performance down the drain; I had ample RAM so that wasn't a concern, and could create a swap file on-the-fly as needed.
I went half-and-half with my 8 GB USB stick, which ended up being 3.7 GB for NTFS and 3.7 GB for Btrfs (it doesn't quite add up, but whatever). On Btrfs, I have 2 GB of OS-related stuff and 2.1 GB free (by magic of compression, 2 GB of OS fits in 1.6 GB of storage
... not bad). Even if you didn't care about compression and preferred a more mature/reliable filesystem such as Ext4fs, there'd be some 1.7 GB of space left for applications.
However, small-block read/write performance is important for both Lubuntu and PortableApps; see this thread
for some relevant discussion about MicroSD performance. It could very well make or break the experience for you; but give it a try and see how it works.