Hello oyama,
you can use the "Stiffness Requirements File" in Express by defining the file path in the: Analysis Criteria -> Stiffness Criteria dialog.
This way, you can define a minimum required stiffness (ABD matrix diagonal) for every individual element in your model.
The file format is as follows:
___________________________________
-1
#of Entries (i.e. number of elements you want to define a stiffness target for)
ElId, [dummy], A11, A22, A33, D11, D22, D33
…
-1
____________________________________
A simple example file defining the minimum (required) stiffness for elements with ID 1 through 10 is attached to this post.
Now, during element sizing optimization, these minimum stiffness requirements will be fulfilled (if your design restrictions allows for it; e.g. if max gauge is high enough for your chosen material).
I hope this helps!
-- Bertram