If your question is "when should I use a column supported fixed roof vs a typical API self-supported design (cone, dome or umbrella)" the answer is usually around 60 feet diameter based on the thickness and radius limitations imposed in the standard, but it varies based on the actual design loads.

The answer to your second question seems to be "hire a competent structureal engineer".

Note that none of the typical API fixed roof designs impose radial loads on the tank shell or a plate/ring in the tank center and therefore tension in the shell or compression on a member in the tank center is not typically needed. If your design does create compression in the rafters, you'll need an engineer familiar with those types of structures.