It may be more economical to upgrade to a "higher" level SQL Server Plan than to simply purchase more disk space. The following link shows the various SQL Server Hosting Plans:
If incrementing up to the next highest plan is undesireable, SQL Server database disk space increases are available at the following prices:
25MB Additional Data disk space: $10.00/month
50MB Additional Data disk space: $15.00/month
100MB Additional Data disk space: $25.00/month
300MB Additional Data disk space: $50.00/month
500MB Additional Data disk space: $80.00/month
Note: Each disk space add-on also includes an equal increase in transaction log disk space allowance. Example: Increasing your data disk space by 25MB includes an additional 25MB disk space for your transaction logs.
Please let us know what disk space increase you would like.