There are 2 types storage in Salesforce.
- Data storage : Record data in System and Custom object
- File storage: Attachment data
Each edition has its own storage limit stated here. Sandbox storage is somehow different from the normal edition. There are 3 different types of Sandbox in Salesforce:
|Type||Data Storage Limit||File Storage Limit||Free|
|Full||Same as production org||Same as production org||1 included in Unlimited|
|Configuration Only||500MB||500MB||5 included in Unlimited|
|Developer||10MB||10MB||15 included in Unlimited
1 included in Enterprise
For Apex development, Developer Sandbox should be sufficient. If you are testing integration with larger data set, you may consider getting a Configuration Only sandbox. Unless you need to replicate a full set of data from production org, you should not need the Full sandbox.