Many people use roaches with their lizards, as far as collareds I know some who use them, Also some who have said they do not like them as much as crickets. Guess you have to just try.
I have never used them. As far as size , suppose it would depend on the age/size of the collared. Hatchlings would get tiny roaches, and work up to about an inch, like an adult cricket.
As far as how much an adult trio eats always varies, adults can eat 5-6 crickets each, sometimes a couple less, sometimes a few more, but thats about norm.
Best wishes with your quest, oh a 60 gallon would be just FINE 
Hope ya stick around and show off those lizards when ya get them.
Pretty kids
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