100st sounds too much, but then again, I'm also a novice. However, I'm knitting a sock atm (hopefully it'll turn into two, but I'm getting tired of it..), and the needles are 2,5mm and the yarn is fairly thin, and I used 64st from the top. So 100 sounds a lot, but then again, the socks are pretty tight, but my calves are also huge, so it still sounds like a lot. I've knitted a ribbed beanie that used 104st, though that was on 5mm needles.
I don't think this is helping at all, and I might be totally wrong :/ I'd probably look for a pattern online, and at least use it for size reference, though I guess it also depends on what yarn you're using.