accessing UIImage properties without loading in memory the image

As of iOS 4.0, the iOS SDK includes the CGImageSource... functions (in the ImageIO framework). It's a very flexible API to query metadata without loading the image into memory. Getting the pixel dimensions of an image should work like this (make sure to include the ImageIO.framework in your target):

#import <ImageIO/ImageIO.h>

NSURL *imageFileURL = [NSURL fileURLWithPath:...];
CGImageSourceRef imageSource = CGImageSourceCreateWithURL((CFURLRef)imageFileURL, NULL);
if (imageSource == NULL) {
    // Error loading image
    ...
    return;
}

CGFloat width = 0.0f, height = 0.0f;
CFDictionaryRef imageProperties = CGImageSourceCopyPropertiesAtIndex(imageSource, 0, NULL);

CFRelease(imageSource);

if (imageProperties != NULL) {

    CFNumberRef widthNum  = CFDictionaryGetValue(imageProperties, kCGImagePropertyPixelWidth);
    if (widthNum != NULL) {
        CFNumberGetValue(widthNum, kCFNumberCGFloatType, &width);
    }

    CFNumberRef heightNum = CFDictionaryGetValue(imageProperties, kCGImagePropertyPixelHeight);
    if (heightNum != NULL) {
        CFNumberGetValue(heightNum, kCFNumberCGFloatType, &height);
    }

    // Check orientation and flip size if required
    CFNumberRef orientationNum = CFDictionaryGetValue(imageProperties, kCGImagePropertyOrientation);
    if (orientationNum != NULL) {
        int orientation;
        CFNumberGetValue(orientationNum, kCFNumberIntType, &orientation);
        if (orientation > 4) {
            CGFloat temp = width;
            width = height;
            height = temp;
        }
    }

    CFRelease(imageProperties);
}

NSLog(@"Image dimensions: %.0f x %.0f px", width, height);

(adapted from "Programming with Quartz" by Gelphman and Laden, listing 9.5, page 228)

Swift 3 version of the answer:

import Foundation
import ImageIO

func sizeForImage(at url: URL) -> CGSize? {

    guard let imageSource = CGImageSourceCreateWithURL(url as CFURL, nil)
        , let imageProperties = CGImageSourceCopyPropertiesAtIndex(imageSource, 0, nil) as? [AnyHashable: Any]
        , let pixelWidth = imageProperties[kCGImagePropertyPixelWidth as String]
        , let pixelHeight = imageProperties[kCGImagePropertyPixelHeight as String]
        , let orientationNumber = imageProperties[kCGImagePropertyOrientation as String]
        else {
            return nil
    }

    var width: CGFloat = 0, height: CGFloat = 0, orientation: Int = 0

    CFNumberGetValue(pixelWidth as! CFNumber, .cgFloatType, &width)
    CFNumberGetValue(pixelHeight as! CFNumber, .cgFloatType, &height)
    CFNumberGetValue(orientationNumber as! CFNumber, .intType, &orientation)

    // Check orientation and flip size if required
    if orientation > 4 { let temp = width; width = height; height = temp }

    return CGSize(width: width, height: height)
}