Depends. Duvets are measured before filling. There is some take-up in the finished size depending on the thickness and type of fill. The thicker the fill (comforter vs. quilt) the smaller the finished dimensions.

That said, a duvet cover should be between 2" and 4" smaller than the duvet measurements. When the duvet fits inside tightly instead of swimming around inside, there is much less chance of the darn thing getting all bunched up in there. It looks better, too.

In your case, the 90X90 size is spot on for a thick, comforter-type duvet. For a thinner one, go an inch or two smaller. Some you will find are slightly rectangular. Don't stress about it, but try to stay within the parameters. It will be fine.

Thank you Woodsy!

It's also nice if the duvet covers have ties inside to tie the corners of the duvet to the cover.

I have one that has the ties inside - and they are awesome~

And if it turns out to be too big, they're easy to make smaller, just a long straight line of sewing!

I purchased my last duvet cover from Overstock.com. It was 300 thread count, piped all the way around and reversible. It's about 12" too big width wise and it's fine. No slippage; it stays in place. I paid $29.99; king size + $3 shipping. It got rave reviews....including mine!

And IKEA has a nice selection, too.

I hate duvet covers-it's impossible to fit a twin quilt inside one. You end up trying to get the corners stuffed, and it's very hard to do! I bought my daughter a duvet cover when her bed quilt was getting a little shabby. She ended up using the empty duvet "as is" on her bed.