I want to save a json file with all the application data (something similar to preference) but im not sure what is the limit size, because if the app cant use this file it will not function probably. is this information known beforehand and the OS reserve some space for your app or its based on the size available.
Update: I dont really care about External storage since is not always available in the device and could be changed (SD card) and i could check for internal storage using this but this is not what i want to know, What i want to know if there's a memory size allocated for internal storage for the device ?
If you use Environment.getExternalStorageDirectory() (or Context.getExternalFilesDir() for API level 8 and up) as the place for your json file, then I believe the size will be limited by the available space in the external storage (usually an SD card). For most devices, I believe there are no fixed limits built into Android for external file storage. (Internal storage is a different matter. Device manufacturers can impose quite restrictive limits, perhaps as low as 100MB shared among all applications.)
UPDATE: Note that according to the compatibility definition for Android 2.3 (Section 7.6.1), devices should have quite a bit of memory:
Device implementations MUST have at least 150MB of non-volatile storage available for user data. That is, the /data partition MUST be at least 150MB.
Beyond the requirements above, device implementations SHOULD have at least 1GB of non-volatile storage available for user data. Note that this higher requirement is planned to become a hard minimum in a future version of Android. Device implementations are strongly encouraged to meet these requirements now, or else they may not be eligible for compatibility for a future version of Android.
This space is shared by all applications, so it can fill up. There is no guaranteed minimum storage available for each app. (Such a guaranteed minimum would be worthless for apps that need to store more than the minimum and would be a waste for apps that store less.)
Edit: From the compatibility definition for Android 4.0
Device implementations MUST have at least 350MB of non-volatile storage available for user data. That is, the /data partition MUST be at least 350MB.
From the compatibility definition for Android 4.3
Device implementations MUST have at least 512MB of non-volatile storage available for user data. That is, the /data partition MUST be at least 512MB.
Interestingly, the recommendation that implementations SHOULD provide at least 1GB has stayed the same.