Straight needles are a poor choice for really big projects because -- exactly as you point out -- the number of stitches they hold is limited.
Long circulars are a better choice: they come in lengths up to about 48", and can get even longer if you combine the cables in an interchangeable needle set like the Denise. A good-sized circular can hold hundreds of stitches, depending on the weight of the yarn.
Question is, do you really want all that weight hanging from your arms and sitting on your lap when the one-piece blanket gets to be twin size or larger?
Sounds exhausting and awkward to me.
Best bet might be to make your blanket in modules -- squares, strips or log-cabin style? (Mason-Dixon Knitting has a wonderful chapter on log-cabin blankets.)
I aslso think that a knitted blanket for a queen or king size bed could weigh a ton and cost a fortune to make. Not to mention how long it would take to knit. How about thinking in terms of a coverlet, afghan or throw that just covers the top of the bed? You get the look without the weight, expense and many months of knitting.