I use a small cage for my young juvenile, simply because that is what is convenient for me. i think that a 15 gallon will work fine. as per crickets, i prefer to leave them in the cage over the few days it takes the geckos to eat them, giving the lizards the choice of time and place. HOWEVER, if you do this you must make sure that a food source is available to the crickets i use a carrot or two, replacing them when they have dried out and the crickets also munch on the uneated CGD.
If you do not supply food for the crickets they will chew on your animals late at night, and this is unhealthy for obvious reasons.
I do see a problem with the animal finding crickets in a large naturalistic enclosure because the crickets will hide in the substrate (this is their natural programing.....) i dont have a natural display, maybe its not an issue. i agree with using smaller crickets, just because its another thing you wont have to worry about, and yes you can use CGD everynight. its just up to you how often you want to use it. I go for every other night.
these are simply my practices. with such hardy animals i think you will find a large range of care tactics that work with great success.
Andrew Ellis