as well as maybe measuring baby i woul recomend doing a gauge swatch. Using the yarn you have, and the needles you have do a variety of test swatches about 5x5 inches and see what sort of fabric you get and then count the stitches and rows per inch to find out what gauge you knit at.
Once you have an idea of what you are getting you will be able to better decide which size to make and how/if you need to alter things like the number of stitches you cast on.
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