Question, what is the size of the disks ?
The disk recommendations is about the number of spindle to meet the performance criteria, not capacity. Balance is an I/O intensive database application, so this recommendation prevents people from deploying Balance on a single local drive on an ESX host. (Which runs painfully slow). If you deploy on a disk aggregate that span a shelf of disks, you will be OK.
How much space should I reserve for this VM ?
The default VA size of 200 GB should be more than sufficient for most deployments. I recommend deploying the original size, as a VMware thin VMDK, and expand it later if needed. In general, it is better to start with the smaller size, and grow it. If you pick too large a size, you cannot shrink it back down, and you will be stuck with it. You can also Storage VMotion it to a larger datastore, if needed.
Question, how many objects can the DB keeps before it reaches 205G of space ?
For the size you are looking at, monitoring NFS datastores should fit within the 200 GB. If it is monitoring on a FC infrastructure, you will see more objects, and may hit the limit a few months sooner. Balance monitors its own disk space usage, so the admin receives an alert long before running out of disk space.
Can the customer keep a period of year data ?
By default Balance keeps one year of data. Anything older than one year is automatically purged.
Would there be any performance impact if he keeps a year of data ?
No, all of the sizing recommendations are made with a full year's worth of data in the database. A brand new Balance install will certainly run faster, but a fully loaded one will perform acceptably.