Quote Originally Posted by kellym
Okay, thanks for everyones patience.
I went out and measured my stock Giant post. I measured the back of the seatpost tube to the back of the actual seatpost head. I came up with 25 millimeters. Does that make sense? If I buy a 22 mm (setback) post will that work?
It depends on how your bike was initally set up. Did you have a bike fit? Do you like your current position? Measure your current saddle setback position (provided that you like it and it is the correct position for you). You can do this 1 of 2 ways.

Method 1 - Drop a plumb line from the nose of your saddle. Measure the horizontal distance from your crank center to the plumb line.

Method 2 - Take yard stick or a similar object that is perfectly straight and line it up (vertically) to your crank center. Measure the horizontal distance from the yard stick to the nose of your saddle.

Quote Originally Posted by kellym

The length of the post is 330 millimeters. I can't even see the numbered adjustment marks on the post where I have it set at for my height. Should I go with a 250 mm post or 300 mm post?

One of the websites advised that the more seatpost inside the frame the more stable and strong the post.
That website is correct....the more seatpost inside the frame = more stability. That is why manufacturers mark the limit of the seatpost. If you exceed that mark, you could ruin your bike frame. So it is important to get one that is long enough (but not excessively long because it adds needless weight to your bike). If you are not sure if a 250 or 300 will work, you might want to bring your bike in to your LBS and ask them. Also remember that changing the saddle setback will change the saddle height (for example less setback would mean an increase in saddle height) as well as changing the saddle height will change the saddle setback.

It really comes down to how much you like your current position. If you are happy with it, get a seatpost enough setback and the proper height. If not, then you will have to figure out what your new position is and take the appropriate measurements.

Good luck,