First of all nice species!
I keep all of my Grammostola species at room temperature. They don't need it warm or humid. Dry the substrate out and just keep an open water dish filled up, this supplies enough humidity.
A good indication for size of container etc is 4x the spiderlings leg span. The tank you're keeping it in now will be sufficient probably for the upcoming moult, then you will probably have to rehouse.
Here's a pic of my Grammostola rosea adult female's setup. She's a 4" leg span.
She is in a flat faunarium. Subby, hide, open water dish
