For me it varies on what horse I'm riding and what I'm doing.
If I'm starting colts, doing a lot of arena work or cutting then I keep my stirrups pretty short. I like a little space between my knees and my saddle. For me that really helps me use my calves, and it's not too short, that I still have a deep seat. I have noticed a lot people usually have the opposite problem-riding with stirrups too long. It looks as though they have to grip with their knees and thighs to stay in the saddle.
This is my deal too. I keep my stirrups fairly short when I'm riding colts. That makes it easier to keep my calves close to their sides. I have found that I am more likely to try to brace against my stirrups if they are a touch on the long side. Right now, I am riding a green quarter pony that I have to take my stirrups up a hole for because he is so narrow. If I feel like I'm "reaching" for my stirrups just to keep them on my feet, then I lose some of the security in my seat.
The most important thing you can really do is to find a length that is comfortable, secure, and balanced for
you. What's comfortable for one person may not be comfortable or secure for the next. What you might try is starting out by adjusting them one hole higher than level with the bottom of your foot, then keep shortening them until they feel right. If you go a hole too far and they start feeling too short, then let them back out and you'll know then what is the most comfortable spot for you.
This is about where I keep mine when I'm riding colts.