How to find size of a file before downloading it in iOS 7?

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Make a request using the HEAD method. For example:

NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:url];
[request setHTTPMethod:@"HEAD"];

This request will be identical to a GET but it won’t return the body. Then call 

long long size = [response expectedContentLength];

Complete example with NSURLConnection (works for NSURLSession too of course):

NSURL *URL = [NSURL URLWithString:@"http://www.google.com"];
NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:URL];
[request setHTTPMethod:@"HEAD"];
NSHTTPURLResponse *response;
[NSURLConnection sendSynchronousRequest:request returningResponse:&response error: nil];
long long size = [response expectedContentLength];
NSLog(@"%lld",size);

This is also useful to conditionally download based on the Last-Modified header (assuming that the server sends you that).

if ([response respondsToSelector:@selector(allHeaderFields)]) {
    NSString *lastModifiedString = [[response allHeaderFields] objectForKey:@"Last-Modified"];
}

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