Source

File: wp-includes/rest-api/endpoints/class-wp-rest-attachments-controller.php

protected function check_upload_size( $file ) {
		if ( ! is_multisite() ) {
			return true;
		}

		if ( get_site_option( 'upload_space_check_disabled' ) ) {
			return true;
		}

		$space_left = get_upload_space_available();

		$file_size = filesize( $file['tmp_name'] );

		if ( $space_left < $file_size ) {
			return new WP_Error(
				'rest_upload_limited_space',
				/* translators: %s: Required disk space in kilobytes. */
				sprintf( __( 'Not enough space to upload. %s KB needed.' ), number_format( ( $file_size - $space_left ) / KB_IN_BYTES ) ),
				array( 'status' => 400 )
			);
		}

		if ( $file_size > ( KB_IN_BYTES * get_site_option( 'fileupload_maxk', 1500 ) ) ) {
			return new WP_Error(
				'rest_upload_file_too_big',
				/* translators: %s: Maximum allowed file size in kilobytes. */
				sprintf( __( 'This file is too big. Files must be less than %s KB in size.' ), get_site_option( 'fileupload_maxk', 1500 ) ),
				array( 'status' => 400 )
			);
		}

		// Include multisite admin functions to get access to upload_is_user_over_quota().
		require_once ABSPATH . 'wp-admin/includes/ms.php';

		if ( upload_is_user_over_quota( false ) ) {
			return new WP_Error(
				'rest_upload_user_quota_exceeded',
				__( 'You have used your space quota. Please delete files before uploading.' ),
				array( 'status' => 400 )
			);
		}

		return true;
	}