No There is no exact size it has to be. If you are going to make it match the tanks outside i would do it proportionally correct Thou that is not even required.
Figure out what heater and water pump you will be using. Make the middle section big enough for the heater and and end section big enough for the water pump. The only other thing to consider is Media area. You can actually put Carbon in a filter bag and then use any filter media. i use 100 and 200 micron filter pad. it takes up less room then sponge and holds what it picks up better . as well as i toss it in the washing machine with some bleach and ready to go again.... The Guy building it can prob help you arrange the interior to meet the needs of your tank .