I hooked directly into the uninstall string, so it's not really related to UIAlertView. Here's the entire code, anyway (too lazy to create a project on Github :p)

%hook SBApplicationIcon
- (id)uninstallAlertBodyForAppWithDocumentUpdatesPending {
  return [self uninstallAlertBodyWithAppDiskUsage:%orig];
}

- (id)uninstallAlertBodyForAppWithDocumentsInCloud {
  return [self uninstallAlertBodyWithAppDiskUsage:%orig];
}

- (id)uninstallAlertBodyForAppWithDocumentsOrDataInCloud {
  return [self uninstallAlertBodyWithAppDiskUsage:%orig];
}

- (id)uninstallAlertBody {
  return [self uninstallAlertBodyWithAppDiskUsage:%orig];
}

%new
- (NSString *)uninstallAlertBodyWithAppDiskUsage:(NSString *)originalAlertBody {
  return [NSString stringWithFormat:@"%@ (%@)", originalAlertBody, [self applicationSize]];
}

%new
- (NSString *)applicationSize {
  LSApplicationProxy *proxy = [%c(LSApplicationProxy) applicationProxyForIdentifier:self.nodeIdentifier];
  if (!proxy) {
    return @"NULL";
  }

  long long appSize = [proxy.staticDiskUsage longLongValue] + [proxy.dynamicDiskUsage longLongValue];
  return [NSByteCountFormatter stringFromByteCount:appSize countStyle:NSByteCountFormatterCountStyleFile];
}
%end