you need to read the value of RoamingStorageQuota to know how much data your application can use see here:http://msdn.microsoft.com/en-gb/library/windows/apps/windows.storage.applicationdata.roamingstoragequota
var roamingStorageQuota = applicationData.roamingStorageQuota;
Typically it is not much... but it is not intended to store data apart from application settings and things like that. The idea is that if you application runs with the same accounts on different devices it will have the same state, properties etc.
If your application needs to store some data you should consider using Azure (or another cloud service) to store that data, that will remove all the size constraints of the roaming folder.