Hello and I have a H.Gigas and they definately love moisture. Mine has a cereal bowl for her water bowl plus her cage is almost kept muddy. She loves it plus they are big burrowers and they don't come out much at all. She just molted a couple of weeks ago and I have yet a chance to measure her. She was 6" when I got her so hopefully she is about 7". They are really pretty though.
I borrowed CM's suggestion for putting water saver crystals in the soil for my H. gigas sub-adult. So far it's been working really well. They definitely like their humidity.
Give your spider a deep substrate, too, they're amazing tunnel builders. Mine has no less than six different tunnels in its enclosure, all leading to a chamber on the bottom that you can see by lifting the enclosure.
Have you seen the jars of jelly-like substance sold as "Water Bites" for crickets? Well, you can also go to Home Depot and pick up a huge jug of them in dry form and add them to the substrate of your ts enclosure. They absorb huge amounts of water and slowly release it into the soil, keeping the enclosure humid.
The crystals have been mentioned on many threads here, but if you want to know more than I can tell you, send a private message to Code Monkey, he can tell you the brand name, etc.
I bought my H. gigas on 6-6-02, it was 1 1/4". Its molted twice and is now at least 4 1/2". I fed it pinky's at first and that may have led to its quick growth. I dont feed pink's anymore cause its impossible to clean up the bloody mess without distroying its burrow. I don't get to see it alot outside its burrow, but it did use the side of its tank for a wall, so with a flashlite I can check it out. I'd have to say, its the meanest T. I own. I call it B.A (badass ).
Your gonna love yours.