I am moving up from a 2 ton unit to a 2 1/2 ton unit for my home. What size of an intake is required for that? Would the existing intake for the 2 ton suffice for the 2 1/2 ton?

There is two hundred CFM of air flow difference between these systems, normally you would not have to change the return or inlarge it for this small of a change. The larger the return air grill the less restrictive to air flow it is and more quiet.
What size is your existing return?

The 2-ton intake is fine for the 2 1/2 ton unit.

XenoSapien