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What do I do to make the circumference smaller?
Just leave out one of the purl stitches between the leaves pattern. You'll have 60 sts around. The heel can be easily adapted, and when starting the toe just k1, p1, knit to last 2 sts, ptog, i.e. drop one of the knit sts at the beginning.
What do I do to make the foot longer/shorter but want to keep the star toe?
For all of you with shoe size 10 or larger (I myself wear a 10, sometimes 11 and the Koigu version fit my foot, but sometimes it just needs to be larger or smaller for that matter, right?) knit another 8 rounds of the pattern repeat, or leave it out - or more if needed. Now comes the part where it gets a bit tricky, but not much.
To get the same finishing of the leaves as in the original pattern, you'll have to mirror the toe decreases. I.e. start with p2tog and end with p1, k2 on all needles.
To make the foot larger, you would finish the whole repeat, instead of ending after the first 8 rnds as written in the pattern. Now the last leaf points into the "wrong direction" for the instructions of the star toe as written in the pattern. Just use the following chart for the mirrored toe:
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Click on chart to make it larger
To sum it up: when ending with half a repeat use toe as described in pattern. When making smaller or larger and ending with the whole 16 rnd repeat, use mirrored toe of chart.
Now, go and knit some. You know you want to!