Substrate and enclosure size advice?

[Tina Banana]

My A.Geniculata's just molted recently and i decided it was time for her to get a new and bigger home (cause she's been living in the same box that she was in when i got her at the pet store). She now has a 3-inch legspan and i got her a 11-inch in length critter keeper. Is that too big for her? Because i read that giving a tarantula too much space can make it feel insecure and causes stress.
I also went to the pet store to get some new potting soil but it was out of stock. However the owner suggested me using spagmoss as a replacement, he said it was for keeping frogs but worked fine for tarantulas. Can you guys give me some opinion on this? Is it a fine substitute for my T? Here's some picture of the packaging:

 

[Tgirl1010]

That moss isn't going to be good as a substrate. I do use little bits of it in some of my enclosures. It holds moisture well. I would use some EcoEarth coco fiber. The 11 inch critter keeper will be fine. Just makes sure she has a nice dark hide, and can find her food. Finding food isn't usually a problem with an A genic; they love to eat! You also want to make sure the critter keeper has plenty of substrate to reduce the risks if she happens to fall, and to give her plenty to burrow in. I would fill it to within 3" or 4" of the top.

 

[Sabeth]

11 inches is fine…the general rule for tarantula housing is that width should be twice the T's leg span, and the height about twice the width. I don't know anything about spagmoss, but I know peat moss is good to use. Also EcoEarth (shredded coconut husk), as Tgirl mentioned.

 

[MatthewM1]

It won't make a good substrate replacement but you can use it in your enclosures as decor. I have quite a few slings that have integrated it into their retreats.

On a side note, NEVER use sphagnum moss for a frog.