Traditionally, quilts were never meant to go all the way to the floor or even cover the box spring. So yes I use my quilts all the time to only cover the top of the mattress and extend only a few inches down the sides. Few of my quilts cover my king size mattress. But this is not a hard fast rule and is dictated by personal preference. Many quilters are fine using as I do and many would much prefer the quilt be like a bedspread and extend all the way to the floor and have ample length for a pillow tuck.
As far as extra large quilts fitting the longarm, well that all depends if it is a rack setup or a sitdown set up. With sit down there is no limit to size but with a rack setup you are limited by the size of your rack. I only have a 10 ft rack (120" measured on the outside). I lose approximately 6" for the side supports and an additional 10" or so for my machines width. So the absolute widest I can quilt would be 100". I have loaded wider but when I do I have no room on the sides to get at my machine bed to change bobbins, oil and test tension. Additionally I have no way to put the side clamps on so I won't take on anything wider than 100". Length is no problem. If your longarmer or your longarm has a larger rack you can obviously do wider quilts. I have never seen any rack set up offered wider than 14'. A 14' rack should allow one to very easily quilt a quilt as wide as 150", possibly even a tad more.
As far as washers that also depends on a few variables. Your WM capacity, how high loft batting you used, weight of fully soaked quilt etc. I have no problems washing a 96" x 108" quilt done with a single layer of Hobbs 80/20 in my standard top loading washer. I have to take a similar sized T-shirt quilt done with high loft batting to the laundromat.
Last edited by feline fanatic; 02-19-2016 at 10:12 AM.