Quick question

does the length of circular needles matter? pattern calls for size 7 16" needles. I have size 6 24"

Probably. If your pattern has 16" worth of stitches and you have to stretch it 8 more inches…well, your project won’t fit!

It depends if it’s being knit in the round. If it’s knitted flat but just on circulars, it being bigger shouldn’t matter. If it’s in the round, then what Lisa said above is right on the money. However, your needle sizes as well as length are different. Unless you’ve done that on purpose because you knit loose, that might not work either.

How many sts? They might not fit around, but you may be able to do a magic loop or modified ML to get started.

it says i have to cast on 80 sts, it’s a beret so i’m probably knitting in the round

When not doing ML, the circ circumference needs to be smaller than the project. (24" is most likely larger than the FO and/or person’s head.) You can always squish sts onto the ndl but can’t stretch them if the length is too long.

I knit hats of about 7-80 sts on a 24" needle. I usually use one a size larger to CO, then join and knit the first round with the one I’m going to use. After the first round, you can use a modified magic loop until it’s longer and the stitches will actually fit around the needle fairly well, especially if you increase later. The modified ML is to let the sts spread around the cord from the left tip as far as they go, then loop the right side and begin knitting. Or you can pull out the cord between two sts and push the sts toward the 2 ends.