First, with the POK::Needed_Memory_Size property on the process (partition address space). It will indicate the needed memory size for the process.
You can also specify memory requirements with AADL memory components. You bind a memory component to a partition process component with the Actual_Memory_Binding property. In that case, the properties (Word_Size, Word_Count, ...) of the memory component will be used to generate its address space.