The documentation around adding further storage to Log Insight isn't exactly clear, "You increase the storage space by adding a new virtual disk to the Log Insight virtual appliance. You can add as many disks as you need, and as your environment permits " I don't believe this is entirely true, I read somewhere that a maximum of 2TB can be added to a Log Insight appliance by adding an additional disk/s and not by increasing the current disk, can someone confirm?
Log Insight 3.0 and 3.3 have a maximum of 4TB addressable log storage, on VMDKs with a maximum size of 2TB each. That means you could have two 2TB VMDKs, or four 1TB VMDKs, etc. Once you get to sizes around that point, though, you'll probably want to scale outward with a larger cluster size instead of adding more disk to existing VMs.
(A single >2TB VMDK won't be usable because of the old MSDOS partition table layout. Converting to GPT can solve that nicely, but there's little pressure to convert existing disk: see previous scale-out.)