Halo SleepSack for Almost 9 Month Old

I would get the size Large. It is still the same as the medium and small, but it will fit her for a little longer than the medium will.

The large Halo sleep sack will fit better but...soon she will be standing...
I use 2 footie blanket sleepers, one a size bigger to fit over the other.
Works great for us!

The large is going to be too big on her if she wears a large. Since she isn't close to walking, she doesn't need the legs. Since she will only be sleeping in the sleep sack, she won't need something to crawl around the house in. There is enough room she can crawl in her crib, though.

Both my son and daughter walked early, and I had them in the medium while they were learning to walk. The large is really large.

But no, it won't hinder her sitting up. In fact, both of my kids figured out how to walk in them.

Why dont you put her in some light weight footed pajamas?

My daughter wore sleepsacks and still does. I have the XL for her now and she is 2 1/2. She is 36" long and about 29 lbs and the thing is huge on her. If anything, I would stick with the medium or large. My daughter was a chunky baby and was able to sit up and pull herself up in the large sleep sack despite not having her feet exposed. I would be very nervous about the extra large size of the sleepsack.

I would get blanket sleepers (with legs), you can put a lightweight cotton sleeper under for more warmth if needed.. It is time to have her legs free so when she is ready to crawl, sit and stand she is free to do so.