Duvets come in all kinds of sizes and this can create headaches for you trying to find the right size duvet insert comforter. Typically we suggest fitting the duvet insert to be equal to or slightly larger then the duvet. Technically you have plus or minus two inches in either dimension). This assumes your duvet cover will get washed - and likely shrink about 5%.
Most catalog companies use a duvet cover in these size ranges:
Most duvet covers will have a legal and country of origin label. On this label they will list out the size (if not simply have a partner help you stretch the fabric and measure with a tight pull on the fabric).
Most duvets and comforters are designed to offer more coverage side to side versus head to foot. So if you see a duvet size listed as 86 x 102 - you need to understand that the product is placed with the widest point being the left to right placement (and the shorter length sold foot to head (This applies to any size bigger then twin xl).