How can I get the size of a folder on SD card in Android?

Just go through all files and sum the length of them:

/**
 * Return the size of a directory in bytes
 */
private static long dirSize(File dir) {

    if (dir.exists()) {
        long result = 0;
        File[] fileList = dir.listFiles();
        if (fileList != null) {
            for(int i = 0; i < fileList.length; i++) {
                // Recursive call if it's a directory
                if(fileList[i].isDirectory()) {
                    result += dirSize(fileList[i]);
                } else {
                    // Sum the file size in bytes
                    result += fileList[i].length();
                }
            }
        }
        return result; // return the file size
    }
    return 0;
}

NOTE: Function written by hand so it could not compile!

Here's some code that avoids recursion, and also calculates the physical size instead of the logical size:

public static long getFileSize(final File file) {
    if (file == null || !file.exists())
        return 0;
    if (!file.isDirectory())
        return file.length();
    final List<File> dirs = new LinkedList<>();
    dirs.add(file);
    long result = 0;
    while (!dirs.isEmpty()) {
        final File dir = dirs.remove(0);
        if (!dir.exists())
            continue;
        final File[] listFiles = dir.listFiles();
        if (listFiles == null || listFiles.length == 0)
            continue;
        for (final File child : listFiles) {
            result += child.length();
            if (child.isDirectory())
                dirs.add(child);
        }
    }
    return result;
}