heatcord under the tub under a timer for the duration you want it on and thermo to the temp you need it at. as long as he has room to stretch out (click clack length approximately 1/2 total length of the reptile) then he can stay in there as long as needed. many differing opinions on that, some people prefer tubs others prefer larger display units. just be sure to increase the size of click clack to display enclosure slowly so your reptile isnt stressed by the sudden feeling of exposure.
Different snakes grow at different rates, I think as soon as it cant stretch its body out to full lenght is when you upgrade. For heating you can get a heat mat, or get two large tiles and a heat cord, coil the heat cord around on the bottom of one tile ( the cord needs to be 2cm apart from other parts of the coil) then glue the two tiles together. I dont think I explained this well enough if you want some pictures I could post them up.
There heaps of different ways to heat it
1 would be getting a heat mat and a thermostat and set it to 31 deg and sit a tile on that then sit the tub on that or if your going to heat 2-3 tubs get a plank of wood and router 2 lines in it and then sit a heat cord in the bits you routed out and put that on a thermostat and sit the tubs on that