I agree with suzeeq. Don't adjust anything about the pattern until you've tried different needle sizes to achieve the CORRECT GAUGE from the get-go.
Patterns can be modified to make the pieces larger or smaller, but FIRST try to get the correct gauge.
I've made modifications to my garment pieces from time to time when the sizes given are not exactly the size I need. For example, I think the Central Park Hoodie large sizes jump from 44" to 48". I needed 46". So I used a larger 'gauge' on purpose. I used one needle bigger than the 'perfect gauge needle' and my CPH came out as a size 46". For a 'zippered jacket' this was perfect. I have room to wear other sweaters underneath without looking like a stuffed penguin!
With another sweater, I needed more room in the 'caboose'...so after I got 'perfect gauge' on my swatch...I cast on 'EXTRA' stitches for the BACK, on purpose, but had to remember to decrease those extra stitches out before getting to the armhole shaping. By the time I was ready to begin the armholes, the stitch count was back to normal.
Anyways, the point is, you CAN modify a sweater, but ALWAYS try to get the right gauge to start with. Let us know if you need more help along the way.
And I totally agree with suzeeq: when making your swatch, cast on MORE STITCHES than needed, and knit for the full 4" or more...then measure your gauge stitches from the middle out. Like, if my required gauge is 17st=4"...I will cast on 21 st...and then measure the center 17st and HOPE they measure 4" exactly. If not, rip the swatch out, and try a different needle.
Swatching is the most critical part of ensuring that the garment will actually FIT RIGHT!