I have an item to sell that is very large 85x50cm (a carrycot). It's on for local pick up at the moment but I know my chances of selling it are much higher if I can post it. Where do I get a box big enough?!
Up to you but personally I would never bother posting large items unless they are extremely high value, or rare items. The postage is generally prohibitive to further-afield buyers, remember, and there will likely be plenty of carrycots on Ebay
it will definitely sell for more with courier option it is not that heavy well under 15/20kg weight limit if under £50 in value will be able to send to most places for about £7-8 ( not northern ireland or scottish highlands or islands mke sure you specifiy that in listing as courier to these places will be nearer £20
easiest things is to get 2 large boxes like big crisp boxes and open out to make box of right size your local supermarket will give them for free as they have to pay the recycling cardboard team to take them away