How to restrict user not to access file ,picture in azure.

Question

  i have azure storage account,in which I upload download files etc,that can be access by via,web or app user,I just want that user only access that files etc,no one else can not access these file,even he has url of these files?

 

Second Question, I know that blob are fixed in size,my files sizes are different in size,will this possible that each time a file uploaded we check its size and create blob suitable for this,rather then a fixed size blob that may waste of memory?