Depends on the look and feel you're going for. A twin comforter will be a bit shorter than a twin XL, obviously. If you don't pull the cover all the way up when making your bed (or don't make your bed) that shouldn't be a problem. Unless you're tall, you'll be well-covered by your blanket. Queen will be longer head-to-toe, and will also drape over the sides of your bed by quite a bit. Based on the height of your bed and the specific dimensions of the blanket, it might even reach the floor. Some people like that look, and love the feel of all the extra cover. I personally find it excessive, but I sleep really hot and don't need all that extra. You could also consider a full comforter, for a sort of happy medium between these two. More on reddit.com

I’ve had twin xl beds for my Kids for the past 15 yrs or so. Have a look at Full or Double bedding for comforters- it just fits them better and you’ll have far more selection More on reddit.com

Dark gray cotton fill comforters are definitely going to be difficult to find. Comforters consist of a shell and fill. So when you search, it may have a 100% cotton shell but a polyester fill. 100% cotton fill comforters are not common and not really recommended because while they are breathable cotton absorbs moisture and can get nasty really easily from sweat. Additionally, finding colors for this type of comforter with such a unique fill is going to be difficult. The easy way out is definitely to buy the 100% cotton duvet and use a dark gray duvet cover. If you’re open to other fills, you should be able to do a comforter with the color you want with the cotton shell. More on reddit.com

I bought my kids twin xl beds. You will have the easiest time buying bedding when the back to school stuff hits the shelves. All the stuff geared for college kids is twin xl. Even the better department stores will come out with higher er end versions of things in xl at that time. You can still find some twin xl bedding from Wamsutta on eBay and such. It’s not a popular size, so the prices haven’t gotten too crazy yet. And for blankets, you can use full (which is big enough to tuck in) also. Many throws come in 70 or 80 inch lengths too- which is perfect for this size bed More on reddit.com