Well if you do a gauge swatch, you can have a better idea of what size you'll end up with. If you're using a worsted weight yarn and a size 15 needle, that will be a VERY loose knit. Is that what you're wanting? I'm doing a worsted weight afghan on size 8 needles and with a 200 stitch cast on, it is working out to be about 45" across.
Say you do a swatch and get, for example, 4sts/1". Then you can figure that if you cast on 200sts you would wind up with 50" across. You can adjust your cast on up and down to get whatever finished size you want.
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