I presume this thread is referring to sending items to Amazon via FBA, using UPS?

This information should be useful to you:

Boxes that contain one or more standard-size items must comply with the following dimensions and weight requirements: 

- Boxes must not measure less than 15.2 cm x 10.0 cm x 2.5 cm and must not exceed 63.5 cm on any side.

- Boxes must not weigh less than 0.15 kg and more than 23 kg.Important to note: 

Any box that weighs more than 15 kg must have a “Team Lift”/”Heavy Package” safety label displayed on the top and sides of each heavy-weight box (5 labels in total).

The article below gives you all the knowledge you need for sending a product to Amazon FBA.

Shipping and routing requirements

Also regarding charges to UPS, we are charged £2.80/£3.50 per box on average.

I would recommend getting boxes which are the maximum size allowed to get the most value out of the box, 24x24x24 (61cmx61cmx61cm)

Personally we use 24x18x18 (61x45.7x45.7) if that helps at all but these boxes suit us better as a business.