Best needle size for scarf

I’m knitting a scarf made of wool with a bulkiness of 5. It’s varigated yarn and I’m using a size 7 needle. It’s looking great…but I was wondering if I should be using a bigger needle. I’m doing it in stockinette stitch.
I’m using a size 9 needle on a similiar bulky yarn for a quick Holiday scarf. I’m just knitting it all the way…but I tend to like how it is turning out.
Besides the suggested needle size on the yarn wraps, is there another way to figure out if other sized needles could/should be used for a project?

It’s important for any project that you like the feel of the knitted fabric that you are making. The needle size on the yarn wrap is a recommendation but you can adjust that as you prefer to get a fabric (and gauge) that you like. For most projects you’ll be given a gauge (stitches/inch) in order to produce the size desired. (which may be more critical for a sweater than for a scarf). You can adjust the needle size in order to obtain the sts/inch recommended. Or you can adjust the pattern to the gauge you like.
Size 7 needles are a little small for a bulky yarn but again, if you like the way the scarf is turning out, then that’s just fine. Here’s a chart that shows you recommended needle sizes for a give yarn weight.

The needle size on the label us mostly to classify the yarn into a weight range. Patterns sometimes use a different one depending on the item. For something like a bulky scarf you can go up a couple of sizes. So that’s a pretty small needle unless you knit loose, a 10 or 11 would be better.

One thing you need to know about stockinette stitch is that it curls under and doesn’t lie flat. If that’s okay with you then great. If you don’t like it then you need to knit an edging on it - 4 rows of garter st where you knit every row, and about 3 sts in garter at the beg and end of the rows. On the knit row, just knit all stitches, but on the other rows, k3, purl to the last 3 sts and knit them. That’ll help your edge be flatter.