There is a size and weight limit for EMS shipment. However, it varies depending on the country/region of destination.
For example, here are the size and weight limit for shipping to the United State of America.
When creating shipping label for shipment to the US, if the weight of parcel exceed 30kg, the error message will be displayed.
For more details, please check the Japan Post page.
Note: When you create EMS shipping labels, a set of necessary documents for the destination country will be issued. The required documents vary depending on the country of destination. Please refer to the Japan Post page for the full list.