I have a older double wide on afull basement. I would like to put a beam in the center where the original roofand ceiling come together. The space is 24' wide and the span is 24', What sizea beam do I need?

There are Beam Calculators online, but can you explain this a little more?
Do you want a beam across the open ceiling, or across the cellar? What is the length of the house? What will be its purpose?

You need the weight the beam will support. What are the dead and live loads?