thermal analysis

Hi 
My question is on what basis do we decide 

1. The dimensions of the sample 
2. The dimensions of the hot and the cold plate 
3. The type of temperature detector sensors used 
4. Heating Source for the hot plate

Hi mechanicians:

I decided to post because I have an inconvenience that is not shown in other posts.

I am working in the Abaqus - Fortran environment in a thermal analysis. After implementing a new procedure (which actually works), the .sta file shows me that it has problems in its unique step, more or less in the 60th iteration (it is supposed to get until 500). Twenty iterations after that gets into interruptions because the code cannot converge.

With these lines it is easier to understand my point:

I've a result from thermal transient analysis. The result shows the maximum and minimum temperature of 4cm * 4cm plate with thickness of 1 mm.

Since the temperature of that plate is not uniform, so I'd like to see the average temperature from overall plate. How can I do it?

Thank you.