Hello LaCrews,
There is no restriction on your QB database size when integrating with Method.
Currently, we do not charge you for storage on Method's server, however in the future you would be allowed 250 MB storage for free and the overage would be charged $0.01 per MB, per day.
To help you track the storage we are looking to implement some sort of measure that would help you to gauge your storage. Keep an eye on our blog for any updates on this.