Tension and adjusting pattern side

i’ve just knitted my tension square. The recommendation is 14sts and 21 rows to 10cm (4in) over st-st on 5 1/2 UK) needles. However I’m normally a very loose knitter so I went down to 4 1/2 size needles to knit the square. My square however was 3/4 in too small, so I’d say that the size 5 1/2 needles are pretty spot on? (I don’t have any at the moment to try)

Also, the largest size on the pattern is 102cm (40in) across the bust. This is the only measurement given as it is a jacket that has one button just below the bust. However I need the jacket to be 122cm (48in) at the bust. The pattern is made up of squares that are sewn together at the end. Therefore, do I increase the size of my needles or add extra stitches to the squares?? The difference between the medium and the large size is 10cm and on average 2 stitches, so would I add 4 stitches if I am going to add stitches?

Thanks,
Mel.b

What you should do is cast on about 24 sts and measure the 10cm over the middle sts and don’t include the side sts. They’re a different size and curl under, so won’t give you a reliable measurement. Then, wash and block the piece like you’re going to for the jacket, and after it’s dry, hang it up. Some yarns relax and grow with washing and with wearing, so you might actually be close enough with your size 4½s, or you may be able to use size 5s.

As far as making it larger, given the construction of it with the squares, it may be easier to use a larger needle, so you may be back to the 5½s.