Easily calculate the bytes used in your Pebble app with the Persistent Storage API. The maximum memory you can use is 4kb.

persistentStorageCalculator

int totalSize = 0;
// Adjust the value (in this example: 2000) to your maximum key that you know you're using in your app
for (int i = 0; i < 2000; i++) {
    if (persist_exists(i)) {
        int size = persist_get_size(i);
        totalSize = totalSize + size;
    }
}
APP_LOG(APP_LOG_LEVEL_DEBUG, "Persistent storage used = %d", totalSize);