No problem
Before we decide what size Drobo volume you are gonna need to know what your storage requirements are. i understand you were intending to keep a copy of your customers video, but are you going to back it up using time machine or just have a single copy of it? and are you open to spending more money on hard discs if the need increases?
You sound like you may be underestimating the amount of storage you are going to need if you are getting into the business of converting/storing VHS and DVD (for example, DVDs are anywhere between 5-9GB each)
I do similar work with video and have 2 x 2TB external drives for storage/workflow (to not clog my boot drive with files!) and a Drobo with 4x2TB drives for time machine backups. It sounds like alot of space but when you are working with large video files or disc images it very quickly disappears!
Because you only have 2 drives in your Drobo right now and they are both the same size you will find only 1TB of it is available over your 2 drives (for protection) which won't be very much space at all if you want to work on video and do full backups of everything.
this is off topic but i do recommend checking out the legalities involved in converting peoples VHS to DVD. i assume it goes without saying that doing this with copyrighted VHS tapes would be illegal but here in the UK there are various data protection and privacy laws which im sure would have an impact on how you stored copies of personal videos (wedding and parties etc) it could seriously come back and bite you in the rear! im sure you would atleast need consent from your customers letting them know why it was stored and for how long