/home quotas are 2G, these are small because /home has the highest level of replication and backup possible.
/projects quotas are 256 GB by default, but you can request that they be increased. At some future date there will be some structure for charging for space over 256GB. /projects is backed up but has slightly less protection than /home.
/lustre/janus_scratch is the Lustre filesystem on Janus and is best for large streaming reads and writes. It is not particularly fast at metadata intense operations (find, du, ls with colors, etc...) and is generally not good with small files and lots of random I/O. True to it's name, janus_scratch has no backups nor does it provide periodic snapshots. At the moment, the /lustre filesystem is to be avoided if at all possible because it can stall execution intermittently for a long time under load from other users, which also makes any timings unpredictable.
/scratch/stmp00 is an NFS scratch area which is better at metadata operations and small files, but is small (8T) and has limited bandwidth (1 GB/s theoretical Max). The filesystem is managed as a normal /tmp filesystem with respect to permissions.