Hi! Since the application form does not have any more questions where you can provide a written answer and expand more on your experiences, I think you will be given a bit more leeway regarding the level of detail you go to into the roles and responsibility section. That said, I do think you will have to observe the same norms of brevity and relevance that apply to cover letter/motivations questions. You can expand on your experiences more than you would in the normal work experience section, provided that (i) everything you write adds value to the recruiter's understanding of your roles, skills, and candidate profile; and (ii) you have been just as diligent in ensuring you have written in a clearly structured and to-the-point manner as you would have been for a section where you had a word limit.
As for the structure, I still advise you to use the normal STAR structure, but perhaps with one or two tweaks to allow you to add details which can show your experiences in a slightly better light:
- Situation: here, instead of just jumping into a description of the general characteristics of the job, you may add one or two sentences describing the broader context: how did you get this job/position of responsibility (eg was it a competitive application process? networking? etc), why did you undertake this role specifically, what is the company/organization you were working for etc.
- Task: here, once again you can expand a bit on the context - where you given this task or did you proactively seek it out? is this the normal type of task someone in your role would be expected to perform, or were you able to take on more high responsibility matters?
- Action: I think this will be similar to how you would write in other work experience section, but perhaps instead of feeling you have to limit yourself at one or two task-action descriptions, you can write about some others if you think they are also relevant.
- Result: here, you will of course want to have a concrete (and if possible, quantifiable) consequence or impact of your action. However, besides that, I think you can also add a few sentences linking the result (and the entire experience more broadly) to your application. Thus, you can link your experience of the role with motivations for commercial law, with motivations for the firm (but make sure you're not going on too much of a stretch!), and with skills/attributes that are useful for a trainee.