If your fenders are pulled out too long, then you might be able to see the top of the fender where the rivets attach it to the stirrup leather coming out from underneath your jockey like this. This runs a very real possibility of pinching your thighs while you ride.
This is usually paired with the fender being way too far through the stirrup so that there is no place to attach the stirrup hobble like this.
On the other hand, sometimes the fender is pulled too far up into the saddle and that leaves a very large length of narrower leather at the bottom of the fender like this.
That puts your buckles (Blevins or otherwise) way too close to the stirrup (as pictured below) and runs the risk of it being uncomfortable for either you or your horse and it also might put them in the way of properly attaching your stirrup hobbles. It could also rub up against either your cinch or other debris on the trail and possibly come undone.
Another huge mistake I see all the time is people who don't have stirrup hobbles on their saddles. The reason that these things are so important is that it keeps your stirrup stable and secure at the bottom of your fenders and keeps the stirrups from turning upside down as seen in the video below. Without a hobble, it is a real possibility for you to end up with a foot between the fender and the leather on a bucking horse or during a fall resulting in dragging. They also keep everything together so that if your Blevins buckle does come undone or something breaks, it will keep everything mostly in place instead of your stirrup just falling completely off. They are not an expensive piece of tack and you can find them almost anywhere.
Shaped Leather Hobble Stirrup Straps - Horse.com
Straight Leather Hobble Stirrup Straps - Horse.com
If you are one of those super long or even super short legged people who either end up with fenders that look like this after being let out to the very last hole
Or like this after being pulled up to the very last hole and maybe punching one or two more.
They do make fenders/leathers in all lengths and styles so that you can find something to fit you. If you have really short legs, you might consider getting a pair of petite or child fenders, if you are extremely leggy, you might look for longer ones.
Now, as for how to fix the appearance of the first two issues. The leathers are either run around the bar/side of the tree itself or through openings carved in the side of the tree. The leather itself is designed to slide across the tree as you adjust your stirrups though sometimes it can be very difficult to actually get it moving. The first thing that you need to do is to remove the stirrup hobble, that will just give you a bit more room and leeway to get your hands in there and pull on the side that needs to be pulled on. Here is a video that pretty well shows what I am talking about.
If they are pulled out too far like you see on the first group of saddles above, then you need to reach behind and pull on the part of the leather with the holes on it. Sometimes you kinda have to wiggle it back and forth while you pull to make it easier. If there is too much of the fender pulled up to the tree, then you want to pull on the fender itself to make it longer and the stirrup leather shorter.
Ideally on your typical western saddle, you should have about 2 to 4 inches (depending on the style of your fender) between the top of your stirrup and where the stirrup leather widens into the fender itself. That gives you enough room to have the ankle mobility without effecting the wider part of the fender but doesn't cause the problems of a too-short fender.
This is what it should look like when everything is properly adjusted.
For information on one way to properly pre-turn your western stirrups, see this thread.
Turning Western Stirrups