These sizes can vary by manufacturer, so you'd best check the description of any of one that you may want to buy. The sizes are usually listed on the package, or in the description, if ordering online.
Twins usually are around 68 x 90.
Full sizes can range from 78 x 86 to 86 x 86.
Queens range from 86 x 86 to 90 x 94. I just purchased a queen comforter last week from Overstock that was 90 x 92.

That's what I'm afraid of. I have looked at the on-line descriptions and it doesn't say - just full/queen. It's from Macy's and they only have the duvet at the store not the comforter. I figured I'd get the comforter since they are the same price and otherwise I need to buy some type of comfortwe to put in it.

Since it says Full/Queen in the description, I would guess that it's probably going to be at the lower size level, (86 x 86), than larger....

That's only my guess, though, and you'd have to order it to be sure. If the size isn't right, couldn't you return it.?

When I was shopping for comfoters, I found that many stores, both online or brick and mortar would check comforter measurements for me. Some manufacturers' full size was larger than other manufacturers' queen size!

Full/queen is usually 88x88, but really there is no way to be exact--manufacturers do what manufacturers do and if a material is just so wide they will make it that size.

full/queen is smaller than queen size. It is just what it says a cross between full and queen

Thanks everyone. I'll call the Macy's store closest and ask if they could measure it for me. Now I'm wondering if I should get the duvet. I really love this one and I've been looking for a year. If needed it would be much easier to cut down the wide of a duvet rather than a comforter. I just don't understand why they are the same price.