Converting Patterns Confusion

I recently came across a scarf pattern that asked for the following:

123yds of super chunky yarn on #19 circular needles.
Casting on 100 stitches.

However, I purchased bulky yarn and #17 circular needles.

I am just curious if there is a specific way to convert this so that I end up with the right length (5 feet). Mind you the scarf pattern is to be knit lengthwise hence the 100 stitches. I have crocheted lengthwise before but never knit and that’s where the confusion comes from. How can I tell when it is long enough on circular needles?

What is the name of the pattern? And a link if possible? Is it garter stitch?

Since it’s a scarf it isn’t critical, but you do want it long enough. I think your yarn and needles will make it smaller, but without yarn info it’s hard to know for sure. You could just cast on extra stitches, but since I don’t know the pattern…