sizing of a pumphouse manifold

With the small amount of information you have provided we are left to pure guess work.

I assume that the Process Engineer has produced a P&ID showing the complete system. If not you are not able to do anything.

If there is a P&ID then it should should show the suction line from the source (a tank or vessel of some type). This suction line should have a size. That size should be carried always as close to the pump as possible. The suction line, the manifold joining the two or more pumps on that service, and the branch to each pump should all be the same size. The pump suction block valve should be this same size.

The pump suction nozzle will normally be one or more sizes smaller than the main suction line size. The reduction should be right at the pump.

We cannot discuss the support of these lines until you tell us if these lines are coming from an elevated source or storage tanks on grade.

Please give us more information.