Gauge

I tend to knit small… I recently have been working on a sandal pattern and was aiming for a 4 inch baby sandal. The final product ended up being 3.5 inches… I always knit small. In the past, I have gone up a needle size to adjust for it…

Im wondering what would be the best strategy when I start my next pair of sandals (its okay because I needed some 3.5 inch sandals anyways)… Should I go up a needle size or try to adjust the pattern to make the sandal a bit bigger?

Depends.

Did you like the weave of the 3.5" sandal? If so, use the same size needle but adjust (CO extra sts, work a few more rows).

If the weave was a bit tight and you would like it to be looser, go up a needle size. Work the pattern as written and see what size you get. Then, adjust further as needed.

I don’t know which pattern you’re using but you might also be able to go up to the next heavier weight yarn