Do you mean that you don't want to buy a size of needle you don't have? You have more stitches in the same space, so they are smaller, so you need more stitches than the pattern states, so a larger size. But I really recommend working out the maths. You don't know whether to go one size up, or two sizes, or inbetween, or what.
What measurement do you want at the bust/waist etc.? At 4.5spi, how many stitches will be required to make a piece of fabric that wide? That's the best number of stitches. Plus two for seams, if knitted flat and then sewn.