The Tenancy Agreement is a two way commitment with obligations and responsibilities on both sides.
No tenant should be in a postion where they feel that they can't complain about the property or tenancy. Whether it's a major or a seemingly trivial issue, any tenant should not be discouraged from complaining to the landlord. This time it's a bath, and if as you say the tenant risks eviction so should not complain, then your advice will be exactly the same for the next time ... the next faulty light switch, the next roof leak the next insecure window, then next time the heating goes out. What you are saying is no tenant should ever complain, which is wrong.
I find it odd that a landlord would specify a smaller bath. I find it more likley that the builder has just thrown a smaller bath in for one of several potential reasons.
The actual circumstances need to be determined, and that can only be once the landlord is made aware of the tenant's complaint. Then if things can't be sorted out amicably, and the issue is a landlord's obligation, the tenant has the option of doing something or nothing.