What would be needed to turn this into "This bathroom is spacious"? Is it simply a definite article ie هذا الحامد instead of هذا حمام?
Yep: هذا الحمام واسع would be this bathroom is spacious
I am confused between جاب and ج'ب. Which one is used where?
Your question is unclear. What is جاب and ج‘ب ?