It's worth noting that there are huge differences in the feature set of AirWave 7.x and AirWave 8.x. Features like Clarity Live, Traffic Analysis, and other feature changes have increased disk utilization.
The RAID10 requirement is a conservative suggestion to ensure that in the event of a disk failure, that the application is able to proceed while the hardware issue is addressed. You could proceed with a single disk if you're willing to risk a single point failure. If you choose the single disk route, you'll want to setup the external backup transfer destination in case of any sort of failure. AirWave is a disk read/write intensive application, so having the available disk I/O is a general concern.
Additional things for disk size consideration:
Any requirements for maintaining historical data?
Types of client behavior? Long term returning clients vs short term one-time visitors - if you're a store or facility that sees a lot of one-time clients, each user entry consumes chunk of data based on historical data requirements.
VisualRF - are the network devices in a single location or spread among several facilities?
Room for network growth?