FEM - aspect ration of element. RM Bridge

Hello,

In FEM Application Guide is stated:

Let us suppose:

-composite structure (steel and concrete)

-small element length in steel part of section (element 5 to 6 - bottom drawing) e.g. smaller than 2 cm

What is acceptable longitudinal subdivision of girder if we have in mind this small length of sheel element?

Have you any advice for modeling such geometry?

Regards