Hi Frogdance.
A common occurrence in the industry is to have a pet shop send you home with little to no instructions, or improper instructions. Glad you found us. The folks here care about your pet as much as you do and will surely help in anyway to get you on the right track to making a healthy and secure home for your frogs.
I would go with a minimum of a 10 gallon but bigger is surely better. If you shopping at Petco, watch for the dollar a gallon sale and take a 20 gallon home for 20 bucks.
The silicone in the aquarium isn't harmful to the frogs so no problem there. Just keep the basics in mind when providing for the frogs. Clean and properly treated water, scheduled maintenance, and the correct temps & humidity.
Ask questions, check out others photo albums for ideas ,and have fun.
Don