The main international standards for standard pallets:

The International Organization for Standardization (ISO) recognizes six pallet sizes that are most commonly used in international trade and listed in ISO 6780:2003. These are the true "international standards."
ISO Standard Pallet Sizes (mm) : 1,200 × 800, 1,200 × 1,000, 1,219 × 1,016, 1,140 × 1,140, 1,100 × 1,100, 1,067 × 1,067.

The most recognized and true "international standard" in Europe and for many international shipments is the EUR-pallet, often marked with the EPAL (European Pallet Association) logo.
Pallet Size: 800 mm x 1,200 mm (31.50" x 47.24")
Why it's the standard: It is the backbone of the European Pallet Pool (EPP) system, where pallets are exchanged between users. If you receive a shipment on an EUR-pallet, you are often expected to return an identical, certified pallet in good condition.

It is the standard for the Grocery Manufacturers Association (GMA) and is designed to optimally fit the standard dimensions of U.S. trucks and shipping containers. It is not part of a formal exchange pool like the EUR-pallet.

Australia uses two main sizes, but the 1,165 mm x 1,165 mm pallet is the most common for the local CHEP and Loscam rental pool systems. The ISO-listed 1,140 mm x 1,140 mm is also used.

Japan & Korea: The 1,100 mm x 1,100 mm size is very common, as it efficiently fits the small, narrow aisles common in their warehouses and stores.
China: While the 1,100 mm x 1,100 mm and 1,200 mm x 1,000 mm are common, China manufactures pallets in all standard sizes for export, with the 48"x40" being a major product for the North American market