sets-size-of-bp-avatar.php

<?php
/**
 * Sets the size of buddypress profile avatar.
 *
 * @param int    $size size in bytes.
 * @param string $type upload type(avatar, cover_upload).
 *
 * @return int
 */
function wbcom_sets_size_of_bp_avatar( $size, $type ) {

	if ( 'avatar' === $type ) {
		$size = 20 * 1024; // change size with your own.
	}

	return $size;
}

add_filter( 'bp_attachments_get_max_upload_file_size', 'wbcom_sets_size_of_bp_avatar', 10, 2 );